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<meta name="DC.Description.Abstract" content="This document defines Remote Authentication Dial In User Service\n(RADIUS) support for the Extensible Authentication Protocol (EAP), an\nauthentication framework which supports multiple authentication\nmechanisms. In the proposed scheme, the Network Access Server (NAS)\nforwards EAP packets to and from the RADIUS server, encapsulated\nwithin EAP-Message attributes. This has the advantage of allowing the\nNAS to support any EAP authentication method, without the need for\nmethod- specific code, which resides on the RADIUS server. While EAP\nwas originally developed for use with PPP, it is now also in use with\nIEEE 802. This memo provides information for the Internet community.">
<meta name="DC.Creator" content="Aboba, Bernard">
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<meta name="DC.Title" content="RADIUS (Remote Authentication Dial In User Service) Support For Extensible Authentication Protocol (EAP)">

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<span class="pre noprint docinfo top">[<a href="http://tools.ietf.org/html" title="Document search and retrieval page">Docs</a>] [<a href="http://tools.ietf.org/rfc/rfc3579.txt" title="Plaintext version of this document">txt</a>|<a href="http://tools.ietf.org/pdf/rfc3579" title="PDF version of this document">pdf</a>] [<a href="http://tools.ietf.org/html/draft-aboba-radius-rfc2869bis" title="draft-aboba-radius-rfc2869bis">draft-aboba-radiu...</a>] [<a href="http://tools.ietf.org/rfcdiff?difftype=--hwdiff&amp;url2=rfc3579" title="Inline diff (wdiff)">Diff1</a>] [<a href="http://tools.ietf.org/rfcdiff?url2=rfc3579" title="Side-by-side diff">Diff2</a>]                 </span><br>
<span class="pre noprint docinfo">                                                                        </span><br>
<span class="pre noprint docinfo">Updated by: <a href="http://tools.ietf.org/html/rfc5080">5080</a>                                           INFORMATIONAL</span><br>
<span class="pre noprint docinfo">                                                                        </span><br>
<pre>Network Working Group                                           B. Aboba
Request for Comments: 3579                                     Microsoft
Updates: <a href="http://tools.ietf.org/html/rfc2869">2869</a>                                                 P. Calhoun
Category: Informational                                        Airespace
                                                          September 2003


          <span class="h1"><h1>RADIUS (Remote Authentication Dial In User Service)</h1></span>
          <span class="h1"><h1>Support For Extensible Authentication Protocol (EAP)</h1></span>

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   This document defines Remote Authentication Dial In User Service
   (RADIUS) support for the Extensible Authentication Protocol (EAP), an
   authentication framework which supports multiple authentication
   mechanisms.  In the proposed scheme, the Network Access Server (NAS)
   forwards EAP packets to and from the RADIUS server, encapsulated
   within EAP-Message attributes.  This has the advantage of allowing
   the NAS to support any EAP authentication method, without the need
   for method-specific code, which resides on the RADIUS server.  While
   EAP was originally developed for use with PPP, it is now also in use
   with IEEE 802.

   This document updates <a href="http://tools.ietf.org/html/rfc2869">RFC 2869</a>.

















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Table of Contents

   <a href="#section-1">1</a>.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-2">2</a>
       <a href="#section-1.1">1.1</a>.  Specification of Requirements. . . . . . . . . . . . . .  <a href="#page-3">3</a>
       <a href="#section-1.2">1.2</a>.  Terminology. . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-3">3</a>
   <a href="#section-2">2</a>.  RADIUS Support for EAP . . . . . . . . . . . . . . . . . . . .  <a href="#page-4">4</a>
       <a href="#section-2.1">2.1</a>.  Protocol Overview. . . . . . . . . . . . . . . . . . . .  <a href="#page-5">5</a>
       <a href="#section-2.2">2.2</a>.  Invalid Packets. . . . . . . . . . . . . . . . . . . . .  <a href="#page-9">9</a>
       <a href="#section-2.3">2.3</a>.  Retransmission . . . . . . . . . . . . . . . . . . . . . <a href="#page-10">10</a>
       <a href="#section-2.4">2.4</a>.  Fragmentation. . . . . . . . . . . . . . . . . . . . . . <a href="#page-10">10</a>
       <a href="#section-2.5">2.5</a>.  Alternative uses . . . . . . . . . . . . . . . . . . . . <a href="#page-11">11</a>
       <a href="#section-2.6">2.6</a>.  Usage Guidelines . . . . . . . . . . . . . . . . . . . . <a href="#page-11">11</a>
   <a href="#section-3">3</a>.  Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-14">14</a>
       <a href="#section-3.1">3.1</a>.  EAP-Message. . . . . . . . . . . . . . . . . . . . . . . <a href="#page-15">15</a>
       <a href="#section-3.2">3.2</a>.  Message-Authenticator. . . . . . . . . . . . . . . . . . <a href="#page-16">16</a>
       <a href="#section-3.3">3.3</a>.  Table of Attributes. . . . . . . . . . . . . . . . . . . <a href="#page-18">18</a>
   <a href="#section-4">4</a>.  Security Considerations. . . . . . . . . . . . . . . . . . . . <a href="#page-19">19</a>
       <a href="#section-4.1">4.1</a>.  Security Requirements. . . . . . . . . . . . . . . . . . <a href="#page-19">19</a>
       <a href="#section-4.2">4.2</a>.  Security Protocol. . . . . . . . . . . . . . . . . . . . <a href="#page-20">20</a>
       <a href="#section-4.3">4.3</a>.  Security Issues. . . . . . . . . . . . . . . . . . . . . <a href="#page-22">22</a>
   <a href="#section-5">5</a>.  IANA Considerations. . . . . . . . . . . . . . . . . . . . . . <a href="#page-30">30</a>
   <a href="#section-6">6</a>.  References . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-30">30</a>
       <a href="#section-6.1">6.1</a>.  Normative References . . . . . . . . . . . . . . . . . . <a href="#page-30">30</a>
       <a href="#section-6.2">6.2</a>.  Informative References . . . . . . . . . . . . . . . . . <a href="#page-32">32</a>
   <a href="#appendix-A">Appendix A</a> - Examples. . . . . . . . . . . . . . . . . . . . . . . <a href="#page-34">34</a>
   <a href="#appendix-B">Appendix B</a> - Change Log. . . . . . . . . . . . . . . . . . . . . . <a href="#page-43">43</a>
   Intellectual Property Statement. . . . . . . . . . . . . . . . . . <a href="#page-44">44</a>
   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-44">44</a>
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-45">45</a>
   Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . <a href="#page-46">46</a>

<span class="h2"><h2><a class="selflink" name="section-1" href="#section-1">1</a>.  Introduction</h2></span>

   The Remote Authentication Dial In User Service (RADIUS) is an
   authentication, authorization and accounting protocol used to control
   network access.  RADIUS authentication and authorization is specified
   in [<a href="http://tools.ietf.org/html/rfc2865" title="&quot;Remote Authentication Dial In User Service (RADIUS)&quot;">RFC2865</a>], and RADIUS accounting is specified in [<a href="http://tools.ietf.org/html/rfc2866" title="&quot;RADIUS Accounting&quot;">RFC2866</a>]; RADIUS
   over IPv6 is specified in [<a href="http://tools.ietf.org/html/rfc3162" title="&quot;RADIUS and IP6&quot;">RFC3162</a>].

   The Extensible Authentication Protocol (EAP), defined in [<a href="http://tools.ietf.org/html/rfc2284" title="&quot;PPP Extensible Authentication Protocol (EAP)&quot;">RFC2284</a>],
   is an authentication framework which supports multiple authentication
   mechanisms.  EAP may be used on dedicated links, switched circuits,
   and wired as well as wireless links.

   To date, EAP has been implemented with hosts and routers that connect
   via switched circuits or dial-up lines using PPP [<a href="http://tools.ietf.org/html/rfc1661" title="&quot;The Point-to-Point Protocol (PPP)&quot;">RFC1661</a>].  It has
   also been implemented with bridges supporting [<a href="#ref-IEEE802" title="ANSI/IEEE Std 802">IEEE802</a>].  EAP
   encapsulation on IEEE 802 wired media is described in [<a href="#ref-IEEE8021X" title="IEEE Std 802.1X-2001">IEEE8021X</a>].



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   RADIUS attributes are comprised of variable length Type-Length-Value
   3-tuples.  New attribute values can be added without disturbing
   existing implementations of the protocol.  This specification
   describes RADIUS attributes supporting the Extensible Authentication
   Protocol (EAP): EAP-Message and Message-Authenticator.  These
   attributes now have extensive field experience.  The purpose of this
   document is to provide clarification and resolve interoperability
   issues.

   As noted in [<a href="http://tools.ietf.org/html/rfc2865" title="&quot;Remote Authentication Dial In User Service (RADIUS)&quot;">RFC2865</a>], a Network Access Server (NAS) that does not
   implement a given service MUST NOT implement the RADIUS attributes
   for that service.  This implies that a NAS that is unable to offer
   EAP service MUST NOT implement the RADIUS attributes for EAP.  A NAS
   MUST treat a RADIUS Access-Accept requesting an unavailable service
   as an Access-Reject instead.

<span class="h3"><h3><a class="selflink" name="section-1.1" href="#section-1.1">1.1</a>.  Specification of Requirements</h3></span>

   In this document, several words are used to signify the requirements
   of the specification.  These words are often capitalized.  The key
   words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
   "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this document
   are to be interpreted as described in [<a href="http://tools.ietf.org/html/rfc2119" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">RFC2119</a>].

<span class="h3"><h3><a class="selflink" name="section-1.2" href="#section-1.2">1.2</a>.  Terminology</h3></span>

   This document frequently uses the following terms:

   authenticator
             The end of the link requiring the authentication.  Also
             known as the Network Access Server (NAS) or RADIUS client.
             Within IEEE 802.1X terminology, the term Authenticator is
             used.

   peer      The other end of the point-to-point link (PPP),
             point-to-point LAN segment (IEEE 802.1X) or wireless link,
             which is being authenticated by the authenticator.  In IEEE
             802.1X, this end is known as the Supplicant.

   authentication server
             An authentication server is an entity that provides an
             authentication service to an authenticator (NAS).  This
             service verifies from the credentials provided by the peer,
             the claim of identity made by the peer; it also may provide
             credentials allowing the peer to verify the identity of the
             authentication server.  Within this document it is assumed
             that the NAS operates as a pass-through, forwarding EAP
             packets between the RADIUS server and the EAP peer.



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             Therefore the RADIUS server operates as an authentication
             server.

   silently discard
             This means the implementation discards the packet without
             further processing.  The implementation SHOULD provide the
             capability of logging the error, including the contents of
             the silently discarded packet, and SHOULD record the event
             in a statistics counter.

   displayable message
             This is interpreted to be a human readable string of
             characters, and MUST NOT affect operation of the protocol.
             The message encoding MUST follow the UTF-8 transformation
             format [<a href="http://tools.ietf.org/html/rfc2279" title="&quot;UTF-8, a transformation format of ISO 10646&quot;">RFC2279</a>].

   Network Access Server (NAS)
             The device providing access to the network.  Also known as
             the Authenticator (IEEE 802.1X or EAP terminology) or
             RADIUS client.

   service   The NAS provides a service to the user, such as IEEE 802 or
             PPP.

   session   Each service provided by the NAS to a peer constitutes a
             session, with the beginning of the session defined as the
             point where service is first provided and the end of the
             session defined as the point where service is ended.  A
             peer may have multiple sessions in parallel or series if
             the NAS supports that, with each session generating a
             separate start and stop accounting record.

<span class="h2"><h2><a class="selflink" name="section-2" href="#section-2">2</a>.  RADIUS Support for EAP</h2></span>

   The Extensible Authentication Protocol (EAP), described in [<a href="http://tools.ietf.org/html/rfc2284" title="&quot;PPP Extensible Authentication Protocol (EAP)&quot;">RFC2284</a>],
   provides a standard mechanism for support of additional
   authentication methods without the NAS to be upgraded to support each
   new method.  Through the use of EAP, support for a number of
   authentication schemes may be added, including smart cards, Kerberos
   [<a href="http://tools.ietf.org/html/rfc1510" title="&quot;The Kerberos Network Authentication Service (V5)&quot;">RFC1510</a>], Public Key [<a href="http://tools.ietf.org/html/rfc2716" title="&quot;PPP EAP TLS Authentication Protocol&quot;">RFC2716</a>], One Time Passwords [<a href="http://tools.ietf.org/html/rfc2284" title="&quot;PPP Extensible Authentication Protocol (EAP)&quot;">RFC2284</a>], and
   others.

   One of the advantages of the EAP architecture is its flexibility.
   EAP is used to select a specific authentication mechanism.  Rather
   than requiring the NAS to be updated to support each new
   authentication method, EAP permits the use of an authentication
   server implementing authentication methods, with the NAS acting as a
   pass-through for some or all methods and peers.



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   A NAS MAY authenticate local peers while at the same time acting as a
   pass-through for non-local peers and authentication methods it does
   not implement locally.  A NAS implementing this specification is not
   required to use RADIUS to authenticate every peer.  However, once the
   NAS begins acting as a pass-through for a particular session, it can
   no longer perform local authentication for that session.

   In order to support EAP within RADIUS, two new attributes,
   EAP-Message and Message-Authenticator, are introduced in this
   document.  This section describes how these new attributes may be
   used for providing EAP support within RADIUS.

<span class="h3"><h3><a class="selflink" name="section-2.1" href="#section-2.1">2.1</a>.  Protocol Overview</h3></span>

   In RADIUS/EAP, RADIUS is used to shuttle RADIUS-encapsulated EAP
   Packets between the NAS and an authentication server.

   The authenticating peer and the NAS begin the EAP conversation by
   negotiating use of EAP.  Once EAP has been negotiated, the NAS SHOULD
   send an initial EAP-Request message to the authenticating peer.  This
   will typically be an EAP-Request/Identity, although it could be an
   EAP-Request for an authentication method (Types 4 and greater).  A
   NAS MAY be configured to initiate with a default authentication
   method.  This is useful in cases where the identity is determined by
   another means (such as Called-Station-Id, Calling-Station-Id and/or
   Originating-Line-Info); where a single authentication method is
   required, which includes its own identity exchange; where identity
   hiding is desired, so that the identity is not requested until after
   a protected channel has been set up.

   The peer replies with an EAP-Response.  The NAS MAY determine from
   the Response that it should proceed with local authentication.
   Alternatively, the NAS MAY act as a pass-through, encapsulating the
   EAP-Response within EAP-Message attribute(s) sent to the RADIUS
   server within a RADIUS Access-Request packet.  If the NAS sends an
   EAP-Request/Identity message as the initial packet, the peer responds
   with an EAP-Response/Identity.  The NAS may determine that the peer
   is local and proceed with local authentication.  If no match is found
   against the list of local users, the NAS encapsulates the
   EAP-Response/Identity message within an EAP-Message attribute,
   enclosed within an Access-Request packet.

   On receiving a valid Access-Request packet containing EAP-Message
   attribute(s), a RADIUS server compliant with this specification and
   wishing to authenticate with EAP MUST respond with an
   Access-Challenge packet containing EAP-Message attribute(s).  If the
   RADIUS server does not support EAP or does not wish to authenticate
   with EAP, it MUST respond with an Access-Reject.



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   EAP-Message attribute(s) encapsulate a single EAP packet which the
   NAS decapsulates and passes on to the authenticating peer.  The peer
   then responds with an EAP-Response packet, which the NAS encapsulates
   within an Access-Request containing EAP-Message attribute(s).  EAP is
   a 'lock step' protocol, so that other than the initial Request, a new
   Request cannot be sent prior to receiving a valid Response.

   The conversation continues until either a RADIUS Access-Reject or
   Access-Accept packet is received from the RADIUS server.  Reception
   of a RADIUS Access-Reject packet MUST result in the NAS denying
   access to the authenticating peer.  A RADIUS Access-Accept packet
   successfully ends the authentication phase.  The NAS MUST NOT
   "manufacture" a Success or Failure packet as the result of a timeout.
   After a suitable number of timeouts have elapsed, the NAS SHOULD
   instead end the EAP conversation.

   Using RADIUS, the NAS can act as a pass-through for an EAP
   conversation between the peer and authentication server, without
   needing to implement the EAP method used between them.  Where the NAS
   initiates the conversation by sending an EAP-Request for an
   authentication method, it may not be required that the NAS fully
   implement the EAP method reflected in the initial EAP-Request.
   Depending on the initial method, it may be sufficient for the NAS to
   be configured with the initial packet to be sent to the peer, and for
   the NAS to act as a pass-through for subsequent messages.  Note that
   since the NAS only encapsulates the EAP-Response in its initial
   Access-Request, the initial EAP-Request within the authentication
   method is not available to the RADIUS server.  For the RADIUS server
   to be able to continue the conversation, either the initial
   EAP-Request is vestigial, so that the RADIUS server need not be aware
   of it, or the relevant information from the initial EAP-Request (such
   as a nonce) is reflected in the initial EAP-Response, so that the
   RADIUS server can obtain it without having received the initial
   EAP-Request.

   Where the initial EAP-Request sent by the NAS is for an
   authentication Type (4 or greater), the peer MAY respond with a Nak
   indicating that it would prefer another authentication method that is
   not implemented locally.  In this case, the NAS SHOULD send
   Access-Request encapsulating the received EAP-Response/Nak.  This
   provides the RADIUS server with a hint about the authentication
   method(s) preferred by the peer, although it does not provide
   information on the Type of the original Request.  It also provides
   the server with the Identifier used in the initial EAP-Request, so
   that Identifier conflicts can be avoided.






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   In order to evaluate whether the alternatives preferred by the
   authenticating peer are allowed, the RADIUS server will typically
   respond with an Access-Challenge containing EAP-Message attribute(s)
   encapsulating an EAP-Request/Identity (Type 1).  This allows the
   RADIUS server to determine the peer identity, so as to be able to
   retrieve the associated authentication policy.  Alternatively, an
   EAP-Request for an authentication method (Type 4 or greater) could be
   sent.  Since the RADIUS server may not be aware of the Type of the
   initial EAP-Request, it is possible for the RADIUS server to choose
   an unacceptable method, and for the peer to respond with another Nak.

   In order to permit non-EAP aware RADIUS proxies to forward the
   Access-Request packet, if the NAS initially sends an
   EAP-Request/Identity message to the peer, the NAS MUST copy the
   contents of the Type-Data field of the EAP-Response/Identity received
   from the peer into the User-Name attribute and MUST include the
   Type-Data field of the EAP-Response/Identity in the User-Name
   attribute in every subsequent Access-Request.  Since RADIUS proxies
   are assumed to act as a pass-through, they cannot be expected to
   parse an EAP-Response/Identity encapsulated within EAP-Message
   attribute(s).  If the NAS initially sends an EAP-Request for an
   authentication method, and the peer identity cannot be determined
   from the EAP-Response, then the User-Name attribute SHOULD be
   determined by another means.  As noted in <a href="http://tools.ietf.org/html/rfc2865#section-5.6">[RFC2865] Section&nbsp;5.6</a>, it
   is recommended that Access-Requests use the value of the
   Calling-Station-Id as the value of the User-Name attribute.

   Having the NAS send the initial EAP-Request packet has a number of
   advantages:

   [<a name="ref-1" id="ref-1">1</a>]  It saves a round trip between the NAS and RADIUS server.

   [<a name="ref-2" id="ref-2">2</a>]  An Access-Request is only sent to the RADIUS server if the
        authenticating peer sends an EAP-Response, confirming that it
        supports EAP.  In situations where peers may be EAP unaware,
        initiating a RADIUS Access-Request on a "carrier sense" or
        "media up" indication may result in many authentication
        exchanges that cannot complete successfully.  For example, on
        wired networks [<a href="#ref-IEEE8021X" title="IEEE Std 802.1X-2001">IEEE8021X</a>] Supplicants typically do not initiate
        the 802.1X conversation with an EAPOL-Start.  Therefore an IEEE
        802.1X-enabled bridge may not be able to determine whether the
        peer supports EAP until it receives a Response to the initial
        EAP-Request.

   [<a name="ref-3" id="ref-3">3</a>]  It allows some peers to be authenticated locally.






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   Although having the NAS send the initial EAP-Request packet has
   substantial advantages, this technique cannot be universally
   employed.  There are circumstances in which the peer identity is
   already known (such as when authentication and accounting is handled
   based on Called-Station-Id, Calling-Station-Id and/or
   Originating-Line-Info), but where the appropriate EAP method may vary
   based on that identity.

   Rather than sending an initial EAP-Request packet to the
   authenticating peer, on detecting the presence of the peer, the NAS
   MAY send an Access-Request packet to the RADIUS server containing an
   EAP-Message attribute signifying EAP-Start.  The RADIUS server will
   typically respond with an Access-Challenge containing EAP-Message
   attribute(s) encapsulating an EAP-Request/Identity (Type 1).
   However, an EAP-Request for an authentication method (Type 4 or
   greater) can also be sent by the server.

   EAP-Start is indicated by sending an EAP-Message attribute with a
   length of 2 (no data).  The Calling-Station-Id SHOULD be included in
   the User-Name attribute.  This may result in a RADIUS Access-Request
   being sent by the NAS to the RADIUS server without first confirming
   that the peer supports EAP.  Since this technique can result in a
   large number of uncompleted RADIUS conversations, in situations where
   EAP unaware peers are common, or where peer support for EAP cannot be
   determined on initial contact (e.g. [<a href="#ref-IEEE8021X" title="IEEE Std 802.1X-2001">IEEE8021X</a>] Supplicants not
   initiating the conversation with an EAPOL-Start) it SHOULD NOT be
   employed by default.

   For proxied RADIUS requests, there are two methods of processing.  If
   the domain is determined based on the Calling-Station-Id,
   Called-Station-Id and/or Originating-Line-Info, the RADIUS server may
   proxy the initial RADIUS Access-Request/EAP-Start.  If the realm is
   determined based on the peer identity, the local RADIUS server MUST
   respond with a RADIUS Access-Challenge including an EAP-Message
   attribute encapsulating an EAP-Request/Identity packet.  The response
   from the authenticating peer SHOULD be proxied to the final
   authentication server.

   If an Access-Request is sent to a RADIUS server which does not
   support the EAP-Message attribute, then an Access-Reject MUST be sent
   in response.  On receiving an Access-Reject, the NAS MUST deny access
   to the authenticating peer.









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<span class="h3"><h3><a class="selflink" name="section-2.2" href="#section-2.2">2.2</a>.  Invalid Packets</h3></span>

   While acting as a pass-through, the NAS MUST validate the EAP header
   fields (Code, Identifier, Length) prior to forwarding an EAP packet
   to or from the RADIUS server.  On receiving an EAP packet from the
   peer, the NAS checks the Code (2) and Length fields, and matches the
   Identifier value against the current Identifier, supplied by the
   RADIUS server in the most recently validated EAP-Request.  On
   receiving an EAP packet from the RADIUS server (encapsulated within
   an Access-Challenge), the NAS checks the Code (1) and Length fields,
   then updates the current Identifier value.  Pending EAP Responses
   that do not match the current Identifier value are silently discarded
   by the NAS.

   Since EAP method fields (Type, Type-Data) are typically not validated
   by a NAS operating as a pass-through, despite these checks it is
   possible for a NAS to forward an invalid EAP packet to or from the
   RADIUS server.  A RADIUS server receiving EAP-Message attribute(s) it
   does not understand SHOULD make the determination of whether the
   error is fatal or non-fatal based on the EAP Type.  A RADIUS server
   determining that a fatal error has occurred MUST send an
   Access-Reject containing an EAP-Message attribute encapsulating
   EAP-Failure.

   A RADIUS server determining that a non-fatal error has occurred MAY
   send an Access-Challenge to the NAS including EAP-Message
   attribute(s) as well as an Error-Cause attribute [<a href="http://tools.ietf.org/html/rfc3576" title="&quot;Dynamic Authorization Extensions to Remote Authentication Dial In User Service (RADIUS)&quot;">RFC3576</a>] with value
   202 (decimal), "Invalid EAP Packet (Ignored)".  The Access-Challenge
   SHOULD encapsulate within EAP-Message attribute(s) the most recently
   sent EAP-Request packet (including the same Identifier value).  On
   receiving such an Access-Challenge, a NAS implementing previous
   versions of this specification will decapsulate the EAP-Request and
   send it to the peer, which will retransmit the EAP-Response.

   A NAS compliant with this specification, on receiving an
   Access-Challenge with an Error-Cause attribute of value 202 (decimal)
   SHOULD discard the EAP-Response packet most recently transmitted to
   the RADIUS server and check whether additional EAP-Response packets
   have been received matching the current Identifier value.  If so, a
   new EAP-Response packet, if available, MUST be sent to the RADIUS
   server within an Access-Request, and the EAP-Message attribute(s)
   included within the Access-Challenge are silently discarded.  If no
   EAP-Response packet is available, then the EAP-Request encapsulated
   within the Access-Challenge is sent to the peer, and the
   retransmission timer is reset.






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   In order to provide protection against Denial of Service (DoS)
   attacks, it is advisable for the NAS to allocate a finite buffer for
   EAP packets received from the peer, and to discard packets according
   to an appropriate policy once that buffer has been exceeded.  Also,
   the RADIUS server is advised to permit only a modest number of
   invalid EAP packets within a single session, prior to terminating the
   session with an Access-Reject.  By default a value of 5 invalid EAP
   packets is recommended.

<span class="h3"><h3><a class="selflink" name="section-2.3" href="#section-2.3">2.3</a>.  Retransmission</h3></span>

   As noted in [<a href="http://tools.ietf.org/html/rfc2284" title="&quot;PPP Extensible Authentication Protocol (EAP)&quot;">RFC2284</a>], if an EAP packet is lost in transit between
   the authenticating peer and the NAS (or vice versa), the NAS will
   retransmit.

   It may be necessary to adjust retransmission strategies and
   authentication timeouts in certain cases.  For example, when a token
   card is used additional time may be required to allow the user to
   find the card and enter the token.  Since the NAS will typically not
   have knowledge of the required parameters, these need to be provided
   by the RADIUS server.  This can be accomplished by inclusion of
   Session-Timeout attribute within the Access-Challenge packet.

   If Session-Timeout is present in an Access-Challenge packet that also
   contains an EAP-Message, the value of the Session-Timeout is used to
   set the EAP retransmission timer for that EAP Request, and that
   Request alone.  Once the EAP-Request has been sent, the NAS sets the
   retransmission timer, and if it expires without having received an
   EAP-Response corresponding to the Request, then the EAP-Request is
   retransmitted.

<span class="h3"><h3><a class="selflink" name="section-2.4" href="#section-2.4">2.4</a>.  Fragmentation</h3></span>

   Using the EAP-Message attribute, it is possible for the RADIUS server
   to encapsulate an EAP packet that is larger than the MTU on the link
   between the NAS and the peer.  Since it is not possible for the
   RADIUS server to use MTU discovery to ascertain the link MTU, the
   Framed-MTU attribute may be included in an Access-Request packet
   containing an EAP-Message attribute so as to provide the RADIUS
   server with this information.  A RADIUS server having received a
   Framed-MTU attribute in an Access-Request packet MUST NOT send any
   subsequent packet in this EAP conversation containing EAP-Message
   attributes whose values, when concatenated, exceed the length
   specified by the Framed-MTU value, taking the link type (specified by
   the NAS-Port-Type attribute) into account.  For example, as noted in
   <a href="http://tools.ietf.org/html/rfc3580#section-3.10">[RFC3580] Section&nbsp;3.10</a>, for a NAS-Port-Type value of IEEE 802.11, the





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   RADIUS server may send an EAP packet as large as Framed-MTU minus
   four (4) octets, taking into account the additional overhead for the
   IEEE 802.1X Version (1), Type (1) and Body Length (2) fields.

<span class="h3"><h3><a class="selflink" name="section-2.5" href="#section-2.5">2.5</a>.  Alternative Uses</h3></span>

   Currently the conversation between security servers and the RADIUS
   server is often proprietary because of lack of standardization.  In
   order to increase standardization and provide interoperability
   between RADIUS vendors and  security vendors, it is recommended that
   RADIUS- encapsulated EAP be used for this conversation.

   This has the advantage of allowing the RADIUS server to support EAP
   without the need for authentication-specific code within the RADIUS
   server.  Authentication-specific code can then reside on a security
   server instead.

   In the case where RADIUS-encapsulated EAP is used in a conversation
   between a RADIUS server and a security server, the security server
   will typically return an Access-Accept message without inclusion of
   the expected attributes currently returned in an Access-Accept.  This
   means that the RADIUS server MUST add these attributes prior to
   sending an Access-Accept message to the NAS.

<span class="h3"><h3><a class="selflink" name="section-2.6" href="#section-2.6">2.6</a>.  Usage Guidelines</h3></span>

<span class="h4"><h4><a class="selflink" name="section-2.6.1" href="#section-2.6.1">2.6.1</a>.  Identifier Space</h4></span>

   In EAP, each session has its own unique Identifier space.  RADIUS
   server implementations MUST be able to distinguish between EAP
   packets with the same Identifier existing within distinct sessions,
   originating on the same NAS.  For this purpose, sessions can be
   distinguished based on NAS and session identification attributes.
   NAS identification attributes include NAS-Identifier,
   NAS-IPv6-Address and NAS-IPv4-Address.  Session identification
   attributes include User-Name, NAS-Port, NAS-Port-Type, NAS-Port-Id,
   Called-Station-Id, Calling-Station-Id and Originating-Line-Info.

<span class="h4"><h4><a class="selflink" name="section-2.6.2" href="#section-2.6.2">2.6.2</a>.  Role Reversal</h4></span>

   Since EAP is a peer-to-peer protocol, an independent and simultaneous
   authentication may take place in the reverse direction.  Both peers
   may act as authenticators and authenticatees at the same time.

   However, role reversal is not supported by this specification.  A
   RADIUS server MUST respond to an Access-Request encapsulating an
   EAP-Request with an Access-Reject.  In order to avoid retransmissions




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   by the peer, the Access-Reject SHOULD include an EAP-Response/Nak
   packet indicating no preferred method, encapsulated within
   EAP-Message attribute(s).

<span class="h4"><h4><a class="selflink" name="section-2.6.3" href="#section-2.6.3">2.6.3</a>.  Conflicting Messages</h4></span>

   The NAS MUST make its access control decision based solely on the
   RADIUS Packet Type (Access-Accept/Access-Reject).  The access control
   decision MUST NOT be based on the contents of the EAP packet
   encapsulated in one or more EAP-Message attributes, if present.

   Access-Accept packets SHOULD have only one EAP-Message attribute in
   them, containing EAP Success; similarly, Access-Reject packets SHOULD
   have only one EAP-Message attribute in them, containing EAP Failure.

   Where the encapsulated EAP packet does not match the result implied
   by the RADIUS Packet Type, the combination is likely to cause
   confusion, because the NAS and peer will arrive at different
   conclusions as to the outcome of the authentication.

   For example, if the NAS receives an Access-Reject with an
   encapsulated EAP Success, it will not grant access to the peer.
   However, on receiving the EAP Success, the peer will be lead to
   believe that it authenticated successfully.

   If the NAS receives an Access-Accept with an encapsulated EAP
   Failure, it will grant access to the peer.  However, on receiving an
   EAP Failure, the peer will be lead to believe that it failed
   authentication.  If no EAP-Message attribute is included within an
   Access-Accept or Access-Reject, then the peer may not be informed as
   to the outcome of the authentication, while the NAS will take action
   to allow or deny access.

   As described in [<a href="http://tools.ietf.org/html/rfc2284" title="&quot;PPP Extensible Authentication Protocol (EAP)&quot;">RFC2284</a>], the EAP Success and Failure packets are
   not acknowledged, and these packets terminate the EAP conversation.
   As a result, if these packets are encapsulated within an
   Access-Challenge, no response will be received, and therefore the NAS
   will send no further Access-Requests to the RADIUS server for the
   session.  As a result, the RADIUS server will not indicate to the NAS
   whether to allow or deny access, while the peer will be informed as
   to the outcome of the authentication.










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   To avoid these conflicts, the following combinations SHOULD NOT be
   sent by a RADIUS server:

      Access-Accept/EAP-Message/EAP Failure
      Access-Accept/no EAP-Message attribute
      Access-Accept/EAP-Start
      Access-Reject/EAP-Message/EAP Success
      Access-Reject/no EAP-Message attribute
      Access-Reject/EAP-Start
      Access-Challenge/EAP-Message/EAP Success
      Access-Challenge/EAP-Message/EAP Failure
      Access-Challenge/no EAP-Message attribute
      Access-Challenge/EAP-Start

   Since the responsibility for avoiding conflicts lies with the RADIUS
   server, the NAS MUST NOT "manufacture" EAP packets in order to
   correct contradictory messages that it receives.  This behavior,
   originally mandated within [<a href="#ref-IEEE8021X" title="IEEE Std 802.1X-2001">IEEE8021X</a>], will be deprecated in the
   future.

<span class="h4"><h4><a class="selflink" name="section-2.6.4" href="#section-2.6.4">2.6.4</a>.  Priority</h4></span>

   A RADIUS Access-Accept or Access-Reject packet may contain EAP-
   Message attribute(s). In order to ensure the correct processing of
   RADIUS packets, the NAS MUST first process the attributes, including
   the EAP-Message attribute(s), prior to processing the Accept/Reject
   indication.

<span class="h4"><h4><a class="selflink" name="section-2.6.5" href="#section-2.6.5">2.6.5</a>.  Displayable Messages</h4></span>

   The Reply-Message attribute, defined in <a href="http://tools.ietf.org/html/rfc2865#section-5.18">[RFC2865], Section&nbsp;5.18</a>,
   indicates text which may be displayed to the peer.  This is similar
   in concept to EAP Notification, defined in [<a href="http://tools.ietf.org/html/rfc2284" title="&quot;PPP Extensible Authentication Protocol (EAP)&quot;">RFC2284</a>].  When sending a
   displayable message to a NAS during an EAP conversation, the RADIUS
   server MUST encapsulate displayable messages within
   EAP-Message/EAP-Request/Notification attribute(s).  Reply-Message
   attribute(s) MUST NOT be included in any RADIUS message containing an
   EAP-Message attribute.  An EAP-Message/EAP-Request/Notification
   SHOULD NOT be included within an Access-Accept or Access-Reject
   packet.

   In some existing implementations, a NAS receiving Reply-Message
   attribute(s) copies the Text field(s) into the Type-Data field of an
   EAP-Request/Notification packet, fills in the Identifier field, and
   sends this to the peer.  However, several issues arise from this:






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   [<a name="ref-1" id="ref-1">1</a>]  Unexpected Responses.  On receiving an EAP-Request/Notification,
        the peer will send an EAP-Response/Notification, and the NAS
        will pass this on to the RADIUS server, encapsulated within
        EAP-Message attribute(s).  However, the RADIUS server may not be
        expecting an Access-Request containing an
        EAP-Message/EAP-Response/Notification attribute.

        For example, consider what happens when a Reply-Message is
        included within an Access-Accept or Access-Reject packet with no
        EAP-Message attribute(s) present.  If the value of the
        Reply-Message attribute is copied into the Type-Data of an
        EAP-Request/Notification and sent to the peer, this will result
        in an Access-Request containing an
        EAP-Message/EAP-Response/Notification attribute being sent by
        the NAS to the RADIUS server.  Since an Access-Accept or
        Access-Reject packet terminates the RADIUS conversation, such an
        Access-Request would not be expected, and could be interpreted
        as the start of another conversation.

   [<a name="ref-2" id="ref-2">2</a>]  Identifier conflicts.  While the EAP-Request/Notification is an
        EAP packet containing an Identifier field, the Reply-Message
        attribute does not contain an Identifier field.  As a result, a
        NAS receiving a Reply-Message attribute and wishing to translate
        this to an EAP-Request/Notification will need to choose an
        Identifier value.  It is possible that the chosen Identifier
        value will conflict with a value chosen by the RADIUS server for
        another packet within the EAP conversation, potentially causing
        confusion between a new packet and a retransmission.

   To avoid these problems, a NAS receiving a Reply-Message attribute
   from the RADIUS server SHOULD silently discard the attribute, rather
   than attempting to translate it to an EAP Notification Request.

<span class="h2"><h2><a class="selflink" name="section-3" href="#section-3">3</a>.  Attributes</h2></span>

   The NAS-Port or NAS-Port-Id attributes SHOULD be included by the NAS
   in Access-Request packets, and either NAS-Identifier, NAS-IP-Address
   or NAS-IPv6-Address attributes MUST be included.  In order to permit
   forwarding of the Access-Reply by EAP-unaware proxies, if a User-Name
   attribute was included in an Access-Request, the RADIUS server MUST
   include the User-Name attribute in subsequent Access-Accept packets.
   Without the User-Name attribute, accounting and billing becomes
   difficult to manage.  The User-Name attribute within the Access-
   Accept packet need not be the same as the User-Name attribute in the
   Access-Request.






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<span class="h3"><h3><a class="selflink" name="section-3.1" href="#section-3.1">3.1</a>.  EAP-Message</h3></span>

   Description

      This attribute encapsulates EAP [<a href="http://tools.ietf.org/html/rfc2284" title="&quot;PPP Extensible Authentication Protocol (EAP)&quot;">RFC2284</a>] packets so as to allow
      the NAS to authenticate peers via EAP without having to understand
      the EAP method it is passing through.

      The NAS places EAP messages received from the authenticating peer
      into one or more EAP-Message attributes and forwards them to the
      RADIUS server within an Access-Request message.  If multiple
      EAP-Message attributes are contained within an Access-Request or
      Access-Challenge packet, they MUST be in order and they MUST be
      consecutive attributes in the Access-Request or Access-Challenge
      packet.  The RADIUS server can return EAP-Message attributes in
      Access-Challenge, Access-Accept and Access-Reject packets.

      When RADIUS is used to enable EAP authentication, Access-Request,
      Access-Challenge, Access-Accept, and Access-Reject packets SHOULD
      contain one or more EAP-Message attributes.  Where more than one
      EAP-Message attribute is included, it is assumed that the
      attributes are to be concatenated to form a single EAP packet.

      Multiple EAP packets MUST NOT be encoded within EAP-Message
      attributes contained within a single Access-Challenge,
      Access-Accept, Access-Reject or Access-Request packet.

      It is expected that EAP will be used to implement a variety of
      authentication methods, including methods involving strong
      cryptography.  In order to prevent attackers from subverting EAP
      by attacking RADIUS/EAP, (for example, by modifying EAP Success or
      EAP Failure packets) it is necessary that RADIUS provide
      per-packet authentication and integrity protection.

      Therefore the Message-Authenticator attribute MUST be used to
      protect all Access-Request, Access-Challenge, Access-Accept, and
      Access-Reject packets containing an EAP-Message attribute.

      Access-Request packets including EAP-Message attribute(s) without
      a Message-Authenticator attribute SHOULD be silently discarded by
      the RADIUS server.  A RADIUS server supporting the EAP-Message
      attribute MUST calculate the correct value of the
      Message-Authenticator and MUST silently discard the packet if it
      does not match the value sent.  A RADIUS server not supporting the
      EAP-Message attribute MUST return an Access-Reject if it receives
      an Access-Request containing an EAP-Message attribute.





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      Access-Challenge, Access-Accept, or Access-Reject packets
      including EAP-Message attribute(s) without a Message-Authenticator
      attribute SHOULD be silently discarded by the NAS.  A NAS
      supporting the EAP-Message attribute MUST calculate the correct
      value of the Message-Authenticator and MUST silently discard the
      packet if it does not match the value sent.

      A summary of the EAP-Message attribute format is shown below.  The
      fields are transmitted from left to right.

       0                   1                   2
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type      |    Length     |     String...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      79 for EAP-Message

   Length

      &gt;= 3

   String

      The String field contains an EAP packet, as defined in [<a href="http://tools.ietf.org/html/rfc2284" title="&quot;PPP Extensible Authentication Protocol (EAP)&quot;">RFC2284</a>].
      If multiple EAP-Message attributes are present in a packet their
      values should be concatenated; this allows EAP packets longer than
      253 octets to be transported by RADIUS.

<span class="h3"><h3><a class="selflink" name="section-3.2" href="#section-3.2">3.2</a>.  Message-Authenticator</h3></span>

   Description

      This attribute MAY be used to authenticate and integrity-protect
      Access-Requests in order to prevent spoofing.  It MAY be used in
      any Access-Request.  It MUST be used in any Access-Request,
      Access-Accept, Access-Reject or Access-Challenge that includes an
      EAP-Message attribute.

      A RADIUS server receiving an Access-Request with a
      Message-Authenticator attribute present MUST calculate the correct
      value of the Message-Authenticator and silently discard the packet
      if it does not match the value sent.






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      A RADIUS client receiving an Access-Accept, Access-Reject or
      Access-Challenge with a Message-Authenticator attribute present
      MUST calculate the correct value of the Message-Authenticator and
      silently discard the packet if it does not match the value sent.

      This attribute is not required in Access-Requests which include
      the User-Password attribute, but is useful for preventing attacks
      on other types of authentication.  This attribute is intended to
      thwart attempts by an attacker to setup a "rogue" NAS, and perform
      online dictionary attacks against the RADIUS server.  It does not
      afford protection against "offline" attacks where the attacker
      intercepts packets containing (for example) CHAP challenge and
      response, and performs a dictionary attack against those packets
      offline.

      A summary of the Message-Authenticator attribute format is shown
      below.  The fields are transmitted from left to right.

       0                   1                   2
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type      |    Length     |     String...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type

      80 for Message-Authenticator

   Length

      18

   String

      When present in an Access-Request packet, Message-Authenticator is
      an HMAC-MD5 [<a href="http://tools.ietf.org/html/rfc2104" title="&quot;HMAC: Keyed-Hashing for Message Authentication&quot;">RFC2104</a>] hash of the entire Access-Request packet,
      including Type, ID, Length and Authenticator, using the shared
      secret as the key, as follows.

      Message-Authenticator = HMAC-MD5 (Type, Identifier, Length,
      Request Authenticator, Attributes)

      When the message integrity check is calculated the signature
      string should be considered to be sixteen octets of zero.







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      For Access-Challenge, Access-Accept, and Access-Reject packets,
      the Message-Authenticator is calculated as follows, using the
      Request-Authenticator from the Access-Request this packet is in
      reply to:

      Message-Authenticator = HMAC-MD5 (Type, Identifier, Length,
      Request Authenticator, Attributes)

      When the message integrity check is calculated the signature
      string should be considered to be sixteen octets of zero.  The
      shared secret is used as the key for the HMAC-MD5 message
      integrity check.  The Message-Authenticator is calculated and
      inserted in the packet before the Response Authenticator is
      calculated.

<span class="h3"><h3><a class="selflink" name="section-3.3" href="#section-3.3">3.3</a>.  Table of Attributes</h3></span>

   The following table provides a guide to which attributes may be found
   in packets including EAP-Message attribute(s), and in what quantity.
   The EAP-Message and Message-Authenticator attributes specified in
   this document MUST NOT be present in an Accounting-Request.  If a
   table entry is omitted, the values found in [<a href="http://tools.ietf.org/html/rfc2548" title="&quot;Microsoft Vendor-specific RADIUS Attributes&quot;">RFC2548</a>], [<a href="http://tools.ietf.org/html/rfc2865" title="&quot;Remote Authentication Dial In User Service (RADIUS)&quot;">RFC2865</a>],
   [<a href="http://tools.ietf.org/html/rfc2868" title="&quot;RADIUS Attributes for Tunnel Protocol Support&quot;">RFC2868</a>], [<a href="http://tools.ietf.org/html/rfc2869" title="&quot;RADIUS Extensions&quot;">RFC2869</a>] and [<a href="http://tools.ietf.org/html/rfc3162" title="&quot;RADIUS and IP6&quot;">RFC3162</a>] should be assumed.

Request  Accept  Reject  Challenge   #    Attribute
0-1      0-1     0       0            1   User-Name
<span class="h2"><h2><a class="selflink" name="section-0" href="#section-0">0</a>        0  </h2></span>     0       0            2   User-Password [Note 1]
<span class="h2"><h2><a class="selflink" name="section-0" href="#section-0">0</a>        0  </h2></span>     0       0            3   CHAP-Password [Note 1]
<span class="h2"><h2><a class="selflink" name="section-0" href="#section-0">0</a>        0  </h2></span>     0       0           18   Reply-Message
<span class="h2"><h2><a class="selflink" name="section-0" href="#section-0">0</a>        0  </h2></span>     0       0           60   CHAP-Challenge
<span class="h2"><h2><a class="selflink" name="section-0" href="#section-0">0</a>        0  </h2></span>     0       0           70   ARAP-Password [Note 1]
<span class="h2"><h2><a class="selflink" name="section-0" href="#section-0">0</a>        0  </h2></span>     0       0           75   Password-Retry
1+       1+      1+      1+          79   EAP-Message [Note 1]
<span class="h2"><h2><a class="selflink" name="section-1" href="#section-1">1</a>        1  </h2></span>     1       1           80   Message-Authenticator [Note 1]
0-1      0       0       0           94   Originating-Line-Info [Note 3]
<span class="h2"><h2><a class="selflink" name="section-0" href="#section-0">0</a>        0  </h2></span>     0-1     0-1        101   Error-Cause [Note 2]
Request  Accept  Reject  Challenge   #    Attribute

   [Note 1] An Access-Request that contains either a User-Password or
   CHAP-Password or ARAP-Password or one or more EAP-Message attributes
   MUST NOT contain more than one type of those four attributes.  If it
   does not contain any of those four attributes, it SHOULD contain a
   Message-Authenticator.  If any packet type contains an EAP-Message
   attribute it MUST also contain a Message-Authenticator.  A RADIUS
   server receiving an Access-Request not containing any of those four
   attributes and also not containing a Message-Authenticator attribute
   SHOULD silently discard it.




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   [Note 2] The Error-Cause attribute is defined in [<a href="http://tools.ietf.org/html/rfc3576" title="&quot;Dynamic Authorization Extensions to Remote Authentication Dial In User Service (RADIUS)&quot;">RFC3576</a>].

   [Note 3] The Originating-Line-Info attribute is defined in [<a href="#ref-NASREQ" title="&quot;Diameter Network Access Server Application&quot;">NASREQ</a>].

   The following table defines the meaning of the above table entries.

   0     This attribute MUST NOT be present.
   0+    Zero or more instances of this attribute MAY be present.
   0-1   Zero or one instance of this attribute MAY be present.
   1     Exactly one instance of this attribute MUST be present.
   1+    One or more of these attributes MUST be present.

<span class="h2"><h2><a class="selflink" name="section-4" href="#section-4">4</a>.  Security Considerations</h2></span>

<span class="h3"><h3><a class="selflink" name="section-4.1" href="#section-4.1">4.1</a>.  Security Requirements</h3></span>

   RADIUS/EAP is used in order to provide authentication and
   authorization for network access.  As a result, both the RADIUS and
   EAP portions of the conversation are potential targets of an attack.
   Threats are discussed in [<a href="http://tools.ietf.org/html/rfc2607" title="&quot;Proxy Chaining and Policy Implementation in Roaming&quot;">RFC2607</a>], [<a href="http://tools.ietf.org/html/rfc2865" title="&quot;Remote Authentication Dial In User Service (RADIUS)&quot;">RFC2865</a>], and [<a href="http://tools.ietf.org/html/rfc3162" title="&quot;RADIUS and IP6&quot;">RFC3162</a>].
   Examples include:

   [<a name="ref-1" id="ref-1">1</a>]  An adversary may attempt to acquire confidential data and
        identities by snooping RADIUS packets.

   [<a name="ref-2" id="ref-2">2</a>]  An adversary may attempt to modify packets containing RADIUS
        messages.

   [<a name="ref-3" id="ref-3">3</a>]  An adversary may attempt to inject packets into a RADIUS
        conversation.

   [<a name="ref-4" id="ref-4">4</a>]  An adversary may launch a dictionary attack against the RADIUS
        shared secret.

   [<a name="ref-5" id="ref-5">5</a>]  An adversary may launch a known plaintext attack, hoping to
        recover the key stream corresponding to a Request Authenticator.

   [<a name="ref-6" id="ref-6">6</a>]  An adversary may attempt to replay a RADIUS exchange.

   [<a name="ref-7" id="ref-7">7</a>]  An adversary may attempt to disrupt the EAP negotiation, in
        order to weaken the authentication, or gain access to peer
        passwords.

   [<a name="ref-8" id="ref-8">8</a>]  An authenticated NAS may attempt to forge NAS or session
        identification attributes,

   [<a name="ref-9" id="ref-9">9</a>]  A rogue (unauthenticated) NAS may attempt to impersonate a
        legitimate NAS.



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   [<a name="ref-10" id="ref-10">10</a>] An attacker may attempt to act as a man-in-the-middle.

   To address these threats, it is necessary to support confidentiality,
   data origin authentication, integrity, and replay protection on a
   per-packet basis.  Bi-directional authentication between the RADIUS
   client and server also needs to be provided.  There is no requirement
   that the identities of RADIUS clients and servers be kept
   confidential (e.g., from a passive eavesdropper).

<span class="h3"><h3><a class="selflink" name="section-4.2" href="#section-4.2">4.2</a>.  Security Protocol</h3></span>

   To address the security vulnerabilities of RADIUS/EAP,
   implementations of this specification SHOULD support IPsec [<a href="http://tools.ietf.org/html/rfc2401" title="&quot;Security Architecture for the Internet Protocol&quot;">RFC2401</a>]
   along with IKE [<a href="http://tools.ietf.org/html/rfc2409" title="&quot;The Internet Key Exchange (IKE)&quot;">RFC2409</a>] for key management.  IPsec ESP [<a href="http://tools.ietf.org/html/rfc2406" title="&quot;IP Encapsulating Security Payload (ESP)&quot;">RFC2406</a>]
   with non-null transform SHOULD be supported, and IPsec ESP with a
   non-null encryption transform and authentication support SHOULD be
   used to provide per-packet confidentiality, authentication, integrity
   and replay protection.  IKE SHOULD be used for key management.

   Within RADIUS [<a href="http://tools.ietf.org/html/rfc2865" title="&quot;Remote Authentication Dial In User Service (RADIUS)&quot;">RFC2865</a>], a shared secret is used for hiding of
   attributes such as User-Password, as well as in computation of the
   Response Authenticator.  In RADIUS accounting [<a href="http://tools.ietf.org/html/rfc2866" title="&quot;RADIUS Accounting&quot;">RFC2866</a>], the shared
   secret is used in computation of both the Request Authenticator and
   the Response Authenticator.

   Since in RADIUS a shared secret is used to provide confidentiality as
   well as integrity protection and authentication, only use of IPsec
   ESP with a non-null transform can provide security services
   sufficient to substitute for RADIUS application-layer security.
   Therefore, where IPSEC AH or ESP null is used, it will typically
   still be necessary to configure a RADIUS shared secret.

   Where RADIUS is run over IPsec ESP with a non-null transform, the
   secret shared between the NAS and the RADIUS server MAY NOT be
   configured.  In this case, a shared secret of zero length MUST be
   assumed.  However, a RADIUS server that cannot know whether incoming
   traffic is IPsec-protected MUST be configured with a non-null RADIUS
   shared secret.

   When IPsec ESP is used with RADIUS, per-packet authentication,
   integrity and replay protection MUST be used.  3DES-CBC MUST be
   supported as an encryption transform and AES-CBC SHOULD be supported.
   AES-CBC SHOULD be offered as a preferred encryption transform if
   supported.  HMAC-SHA1-96 MUST be supported as an authentication
   transform.  DES-CBC SHOULD NOT be used as the encryption transform.






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   A typical IPsec policy for an IPsec-capable RADIUS client is
   "Initiate IPsec, from me to any destination port UDP 1812".  This
   causes an IPsec SA to be set up by the RADIUS client prior to sending
   RADIUS traffic.  If some RADIUS servers contacted by the client do
   not support IPsec, then a more granular policy will be required:
   "Initiate IPsec, from me to IPsec-Capable-RADIUS-Server, destination
   port UDP 1812".

   For an IPsec-capable RADIUS server, a typical IPsec policy is "Accept
   IPsec, from any to me, destination port 1812".  This causes the
   RADIUS server to accept (but not require) use of IPsec.  It may not
   be appropriate to require IPsec for all RADIUS clients connecting to
   an IPsec-enabled RADIUS server, since some RADIUS clients may not
   support IPsec.

   Where IPsec is used for security, and no RADIUS shared secret is
   configured, it is important that the RADIUS client and server perform
   an authorization check.  Before enabling a host to act as a RADIUS
   client, the RADIUS server SHOULD check whether the host is authorized
   to provide network access.  Similarly, before enabling a host to act
   as a RADIUS server, the RADIUS client SHOULD check whether the host
   is authorized for that role.

   RADIUS servers can be configured with the IP addresses (for IKE
   Aggressive Mode with pre-shared keys) or FQDNs (for certificate
   authentication) of RADIUS clients.  Alternatively, if a separate
   Certification Authority (CA) exists for RADIUS clients, then the
   RADIUS server can configure this CA as a trust anchor [<a href="http://tools.ietf.org/html/rfc3280" title="&quot;Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile&quot;">RFC3280</a>] for
   use with IPsec.

   Similarly, RADIUS clients can be configured with the IP addresses
   (for IKE Aggressive Mode with pre-shared keys) or FQDNs (for
   certificate authentication) of RADIUS servers.  Alternatively, if a
   separate CA exists for RADIUS servers, then the RADIUS client can
   configure this CA as a trust anchor for use with IPsec.

   Since unlike SSL/TLS, IKE does not permit certificate policies to be
   set on a per-port basis, certificate policies need to apply to all
   uses of IPsec on RADIUS clients and servers.  In IPsec deployments
   supporting only certificate authentication, a management station
   initiating an IPsec-protected telnet session to the RADIUS server
   would need to obtain a certificate chaining to the RADIUS client CA.
   Issuing such a certificate might not be appropriate if the management
   station was not authorized as a RADIUS client.

   Where RADIUS clients may obtain their IP address dynamically (such as
   an Access Point supporting DHCP), IKE Main Mode with pre-shared keys
   [<a href="http://tools.ietf.org/html/rfc2409" title="&quot;The Internet Key Exchange (IKE)&quot;">RFC2409</a>] SHOULD NOT be used, since this requires use of a group



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   pre-shared key; instead, Aggressive Mode SHOULD be used.  IKEv2, a
   work in progress, may address this issue in the future.  Where RADIUS
   client addresses are statically assigned, either Aggressive Mode or
   Main Mode MAY be used.  With certificate authentication, Main Mode
   SHOULD be used.

   Care needs to be taken with IKE Phase 1 Identity Payload selection in
   order to enable mapping of identities to pre-shared keys even with
   Aggressive Mode.  Where the ID_IPV4_ADDR or ID_IPV6_ADDR Identity
   Payloads are used and addresses are dynamically assigned, mapping of
   identities to keys is not possible, so that group pre-shared keys are
   still a practical necessity.  As a result, the ID_FQDN identity
   payload SHOULD be employed in situations where Aggressive mode is
   utilized along with pre-shared keys and IP addresses are dynamically
   assigned.  This approach also has other advantages, since it allows
   the RADIUS server and client to configure themselves based on the
   fully qualified domain name of their peers.

   Note that with IPsec, security services are negotiated at the
   granularity of an IPsec SA, so that RADIUS exchanges requiring a set
   of security services different from those negotiated with existing
   IPsec SAs will need to negotiate a new IPsec SA.  Separate IPsec SAs
   are also advisable where quality of service considerations dictate
   different handling RADIUS conversations.  Attempting to apply
   different quality of service to connections handled by the same IPsec
   SA can result in reordering, and falling outside the replay window.
   For a discussion of the issues, see [<a href="http://tools.ietf.org/html/rfc2983" title="&quot;Differentiated Services and Tunnels&quot;">RFC2983</a>].

<span class="h3"><h3><a class="selflink" name="section-4.3" href="#section-4.3">4.3</a>.  Security Issues</h3></span>

   This section provides more detail on the vulnerabilities identified
   in <a href="#section-4.1">Section 4.1</a>., and how they may be mitigated.  Vulnerabilities
   include:

   Privacy issues
   Spoofing and hijacking
   Dictionary attacks
   Known plaintext attacks
   Replay attacks
   Negotiation attacks
   Impersonation
   Man in the middle attacks
   Separation of authenticator and authentication server
   Multiple databases







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<span class="h4"><h4><a class="selflink" name="section-4.3.1" href="#section-4.3.1">4.3.1</a>.  Privacy Issues</h4></span>

   Since RADIUS messages may contain the User-Name attribute as well as
   NAS-IP-Address or NAS-Identifier attributes, an attacker snooping on
   RADIUS traffic may be able to determine the geographic location of
   peers in real time.  In wireless networks, it is often assumed that
   RADIUS traffic is physically secure, since it typically travels over
   the wired network and that this limits the release of location
   information.

   However, it is possible for an authenticated attacker to spoof ARP
   packets [<a href="http://tools.ietf.org/html/rfc826" title="&quot;An Ethernet Address Resolution Protocol&quot;">RFC826</a>] so as to cause diversion of RADIUS traffic onto the
   wireless network.  In this way an attacker may obtain RADIUS packets
   from which it can glean peer location information, or which it can
   subject to a known plaintext or offline dictionary attack.  To
   address these vulnerabilities, implementations of this specification
   SHOULD use IPsec ESP with non-null transform and per-packet
   encryption, authentication, integrity and replay protection to
   protect both RADIUS authentication [<a href="http://tools.ietf.org/html/rfc2865" title="&quot;Remote Authentication Dial In User Service (RADIUS)&quot;">RFC2865</a>] and accounting [<a href="http://tools.ietf.org/html/rfc2866" title="&quot;RADIUS Accounting&quot;">RFC2866</a>]
   traffic, as described in <a href="#section-4.2">Section 4.2</a>.

<span class="h4"><h4><a class="selflink" name="section-4.3.2" href="#section-4.3.2">4.3.2</a>.  Spoofing and Hijacking</h4></span>

   Access-Request packets with a User-Password attribute establish the
   identity of both the user and the NAS sending the Access-Request,
   because of the way the shared secret between the NAS and RADIUS
   server is used.  Access-Request packets with CHAP-Password or
   EAP-Message attributes do not have a User-Password attribute.  As a
   result, the Message-Authenticator attribute SHOULD be used in
   Access-Request packets that do not have a User-Password attribute, in
   order to establish the identity of the NAS sending the request.

   An attacker may attempt to inject packets into the conversation
   between the NAS and the RADIUS server, or between the RADIUS server
   and the security server.  RADIUS [<a href="http://tools.ietf.org/html/rfc2865" title="&quot;Remote Authentication Dial In User Service (RADIUS)&quot;">RFC2865</a>] does not support
   encryption other than attribute hiding.  As described in [<a href="http://tools.ietf.org/html/rfc2865" title="&quot;Remote Authentication Dial In User Service (RADIUS)&quot;">RFC2865</a>],
   only Access-Reply and Access-Challenge packets are integrity
   protected.  Moreover, the per-packet authentication and integrity
   protection mechanism described in [<a href="http://tools.ietf.org/html/rfc2865" title="&quot;Remote Authentication Dial In User Service (RADIUS)&quot;">RFC2865</a>] has known weaknesses
   [<a href="#ref-MD5Attack" title="&quot;The Status of MD5 After a Recent Attack&quot;">MD5Attack</a>], making it a tempting target for attackers looking to
   subvert RADIUS/EAP.

   To provide stronger security, the Message-Authenticator attribute
   MUST be used in all RADIUS packets containing an EAP-Message
   attribute.  Implementations of this specification SHOULD use IPsec
   ESP with non-null transform and per-packet encryption,
   authentication, integrity and replay protection, as described in
   <a href="#section-4.2">Section 4.2</a>.



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<span class="h4"><h4><a class="selflink" name="section-4.3.3" href="#section-4.3.3">4.3.3</a>.  Dictionary Attacks</h4></span>

   The RADIUS shared secret is vulnerable to offline dictionary attack,
   based on capture of the Response Authenticator or
   Message-Authenticator attribute.  In order to decrease the level of
   vulnerability, [<a href="http://tools.ietf.org/html/rfc2865" title="&quot;Remote Authentication Dial In User Service (RADIUS)&quot;">RFC2865</a>] recommends:

      The secret (password shared between the client and the RADIUS
      server) SHOULD be at least as large and unguessable as a
      well-chosen password.  It is preferred that the secret be at least
      16 octets.

   The risk of an offline dictionary attack can be further reduced by
   employing IPsec ESP with non-null transform in order to encrypt the
   RADIUS conversation, as described in <a href="#section-4.2">Section 4.2</a>.

<span class="h4"><h4><a class="selflink" name="section-4.3.4" href="#section-4.3.4">4.3.4</a>.  Known Plaintext Attacks</h4></span>

   Since EAP [<a href="http://tools.ietf.org/html/rfc2284" title="&quot;PPP Extensible Authentication Protocol (EAP)&quot;">RFC2284</a>] does not support PAP, the RADIUS User-Password
   attribute is not used to carry hidden user passwords within
   RADIUS/EAP conversations.  The User-Password hiding mechanism,
   defined in [<a href="http://tools.ietf.org/html/rfc2865" title="&quot;Remote Authentication Dial In User Service (RADIUS)&quot;">RFC2865</a>] utilizes MD5, defined in [<a href="http://tools.ietf.org/html/rfc1321" title="&quot;The MD5 Message-Digest Algorithm&quot;">RFC1321</a>], in order to
   generate a key stream based on the RADIUS shared secret and the
   Request  Authenticator.  Where PAP is in use, it is possible to
   collect key streams corresponding to a given Request Authenticator
   value, by capturing RADIUS conversations corresponding to a PAP
   authentication attempt, using a known password.  Since the
   User-Password is known, the key stream corresponding to a given
   Request Authenticator can be determined and stored.

   Since the key stream may have been determined previously from a known
   plaintext attack, if the Request Authenticator repeats, attributes
   encrypted using the RADIUS attribute hiding mechanism should be
   considered compromised.  In addition to the User-Password attribute,
   which is not used with EAP, this includes attributes such as
   Tunnel-Password [RFC2868, <a href="#section-3.5">section 3.5</a>] and MS-MPPE-Send-Key and
   MS-MPPE-Recv-Key attributes [RFC2548, <a href="#section-2.4">section 2.4</a>], which include a
   Salt field as part of the hiding algorithm.

   To avoid this, <a href="http://tools.ietf.org/html/rfc2865#section-3">[RFC2865], Section&nbsp;3</a> advises:

      Since it is expected that the same secret MAY be used to
      authenticate with servers in disparate geographic regions, the
      Request Authenticator field SHOULD exhibit global and temporal
      uniqueness.






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   Where the Request Authenticator repeats, the Salt field defined in
   <a href="http://tools.ietf.org/html/rfc2548#section-2.4">[RFC2548], Section&nbsp;2.4</a> does not provide protection against
   compromise.  This is because MD5 [<a href="http://tools.ietf.org/html/rfc1321" title="&quot;The MD5 Message-Digest Algorithm&quot;">RFC1321</a>], rather than HMAC-MD5
   [<a href="http://tools.ietf.org/html/rfc2104" title="&quot;HMAC: Keyed-Hashing for Message Authentication&quot;">RFC2104</a>], is used to generate the key stream, which is calculated
   from the 128-bit RADIUS shared secret (S), the  128-bit Request
   Authenticator (R), and the Salt field (A), using the formula b(1) =
   MD5(S + R + A).  Since the Salt field is placed at the end, if the
   Request Authenticator were to repeat on a network where PAP is in
   use, then the salted keystream could be calculated from the
   User-Password keystream by continuing the MD5 calculation based on
   the Salt field (A), which is sent in the clear.

   Even though EAP does not support PAP authentication, a security
   vulnerability can still exist where the same RADIUS shared secret is
   used for hiding User-Password as well as other attributes.  This can
   occur, for example, if the same RADIUS proxy handles authentication
   requests for both EAP and PAP.

   The threat can be mitigated by protecting RADIUS with IPsec ESP with
   non-null transform, as described in <a href="#section-4.2">Section 4.2</a>.  Where RADIUS shared
   secrets are configured, the RADIUS shared secret used by a NAS
   supporting EAP MUST NOT be reused by a NAS utilizing the
   User-Password attribute, since improper shared secret hygiene could
   lead to compromise of hidden attributes.

<span class="h4"><h4><a class="selflink" name="section-4.3.5" href="#section-4.3.5">4.3.5</a>.  Replay Attacks</h4></span>

   The RADIUS protocol provides only limited support for replay
   protection.  RADIUS Access-Requests include liveness via the 128-bit
   Request Authenticator.  However, the Request Authenticator is not a
   replay counter.  Since RADIUS servers may not maintain a cache of
   previous Request Authenticators, the Request Authenticator does not
   provide replay protection.

   RADIUS accounting [<a href="http://tools.ietf.org/html/rfc2866" title="&quot;RADIUS Accounting&quot;">RFC2866</a>] does not support replay protection at the
   protocol level.  Due to the need to support failover between RADIUS
   accounting servers, protocol-based replay protection is not
   sufficient to prevent duplicate accounting records.  However, once
   accepted by the accounting server, duplicate accounting records can
   be detected by use of the the Acct-Session-Id [RFC2866, <a href="#section-5.5">section 5.5</a>]
   and Event-Timestamp [RFC2869, <a href="#section-5.3">section 5.3</a>] attributes.

   Unlike RADIUS authentication, RADIUS accounting does not use the
   Request Authenticator as a nonce.  Instead, the Request Authenticator
   contains an MD5 hash calculated over the Code, Identifier, Length,
   and request attributes of the Accounting Request packet, plus the
   shared secret.  The Response Authenticator also contains an MD5 hash
   calculated over the Code, Identifier and Length, the Request



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   Authenticator field from the Accounting-Request packet being replied
   to, the response attributes and the shared secret.

   Since the Accounting Response Authenticator depends in part on the
   Accounting Request Authenticator, it is not possible to replay an
   Accounting-Response unless the Request Authenticator repeats.  While
   it is possible to utilize EAP methods such as EAP TLS [<a href="http://tools.ietf.org/html/rfc2716" title="&quot;PPP EAP TLS Authentication Protocol&quot;">RFC2716</a>] which
   include liveness checks on both sides, not all EAP messages will
   include liveness so that this provides incomplete protection.

   Strong replay protection for RADIUS authentication and accounting can
   be provided by enabling IPsec replay protection with RADIUS, as
   described in <a href="#section-4.2">Section 4.2</a>.

<span class="h4"><h4><a class="selflink" name="section-4.3.6" href="#section-4.3.6">4.3.6</a>.  Negotiation Attacks</h4></span>

   In a negotiation attack a rogue NAS, tunnel server, RADIUS proxy or
   RADIUS server attempts to cause the authenticating peer to choose a
   less secure authentication method.  For example, a session that would
   normally be authenticated with EAP would instead be authenticated via
   CHAP or PAP; alternatively, a connection that would normally be
   authenticated via a more secure EAP method such as EAP-TLS [<a href="http://tools.ietf.org/html/rfc2716" title="&quot;PPP EAP TLS Authentication Protocol&quot;">RFC2716</a>]
   might be made to occur via a less secure EAP method, such as
   MD5-Challenge.  The threat posed by rogue devices, once thought to be
   remote, has gained currency given compromises of telephone company
   switching systems, such as those described in [<a href="#ref-Masters" title="&quot;Masters of Deception.&quot;">Masters</a>].

   Protection against negotiation attacks requires the elimination of
   downward negotiations.  The RADIUS exchange may be further protected
   by use of IPsec, as described in <a href="#section-4.2">Section 4.2</a>.  Alternatively, where
   IPsec is not used, the vulnerability can be mitigated via
   implementation of per-connection policy on the part of the
   authenticating peer, and per-peer policy on the part of the RADIUS
   server.  For the authenticating peer, authentication policy should be
   set on a per-connection basis.  Per-connection policy allows an
   authenticating peer to negotiate a strong EAP method when connecting
   to one service, while negotiating a weaker EAP method for another
   service.

   With per-connection policy, an authenticating peer will only attempt
   to negotiate EAP for a session in which EAP support is expected.  As
   a result, there is a presumption that an authenticating peer
   selecting EAP requires that level of security.  If it cannot be
   provided, it is likely that there is some kind of misconfiguration,
   or even that the authenticating peer is contacting the wrong server.
   Should the NAS not be able to negotiate EAP, or should the
   EAP-Request sent by the NAS be of a different EAP type than what is
   expected, the authenticating peer MUST disconnect.  An authenticating



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   peer expecting EAP to be negotiated for a session MUST NOT negotiate
   a weaker method, such as CHAP or PAP.  In wireless networks, the
   service advertisement itself may be spoof-able, so that an attacker
   could fool the peer into negotiating an authentication method
   suitable for a less secure network.

   For a NAS, it may not be possible to determine whether a peer is
   required to authenticate with EAP until the peer's identity is known.
   For example, for shared-uses NASes it is possible for one reseller to
   implement EAP while another does not.  Alternatively, some peer might
   be authenticated locally by the NAS while other peers are
   authenticated via RADIUS.  In such cases, if any peers of the NAS
   MUST do EAP, then the NAS MUST attempt to negotiate EAP for every
   session.  This avoids forcing a peer to support more than one
   authentication type, which could weaken security.

   If CHAP is negotiated, the NAS will pass the User-Name and
   CHAP-Password attributes to the RADIUS server in an Access-Request
   packet.  If the peer is not required to use EAP, then the RADIUS
   server will respond with an Access-Accept or Access-Reject packet as
   appropriate.  However, if CHAP has been negotiated but EAP is
   required, the RADIUS server MUST respond with an Access-Reject,
   rather than an Access-Challenge/EAP-Message/EAP-Request packet.  The
   authenticating peer MUST refuse to renegotiate authentication, even
   if the renegotiation is from CHAP to EAP.

   If EAP is negotiated but is not supported by the RADIUS proxy or
   server, then the server or proxy MUST respond with an Access-Reject.
   In these cases, a PPP NAS MUST send an LCP-Terminate and disconnect
   the peer.  This is the correct behavior since the authenticating peer
   is expecting EAP to be negotiated, and that expectation cannot be
   fulfilled.  An EAP-capable authenticating peer MUST refuse to
   renegotiate the authentication protocol if EAP had initially been
   negotiated.  Note that problems with a non-EAP capable RADIUS proxy
   could prove difficult to diagnose, since a peer connecting from one
   location (with an EAP-capable proxy) might be able to successfully
   authenticate via EAP, while the same peer connecting at another
   location (and encountering an EAP-incapable proxy) might be
   consistently disconnected.

<span class="h4"><h4><a class="selflink" name="section-4.3.7" href="#section-4.3.7">4.3.7</a>.  Impersonation</h4></span>

   [<a name="ref-RFC2865" id="ref-RFC2865">RFC2865</a>] <a href="#section-3">Section 3</a> states:

      A RADIUS server MUST use the source IP address of the RADIUS UDP
      packet to decide which shared secret to use, so that RADIUS
      requests can be proxied.




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   When RADIUS requests are forwarded by a proxy, the NAS-IP-Address or
   NAS-IPv6-Address attributes may not match the source address.  Since
   the NAS-Identifier attribute need not contain an FQDN, this attribute
   also may not correspond to the source address, even indirectly, with
   or without a proxy present.

   As a result, the authenticity check performed by a RADIUS server or
   proxy does not verify the correctness of NAS identification
   attributes.  This makes it possible for a rogue NAS to forge
   NAS-IP-Address, NAS-IPv6-Address or NAS-Identifier attributes within
   a RADIUS Access-Request in order to impersonate another NAS.  It is
   also possible for a rogue NAS to forge session identification
   attributes such as Called-Station-Id, Calling-Station-Id, and
   Originating-Line-Info.

   This could fool the RADIUS server into subsequently sending
   Disconnect or CoA-Request messages [<a href="http://tools.ietf.org/html/rfc3576" title="&quot;Dynamic Authorization Extensions to Remote Authentication Dial In User Service (RADIUS)&quot;">RFC3576</a>] containing forged
   session identification attributes to a NAS targeted by an attacker.

   To address these vulnerabilities RADIUS proxies SHOULD check whether
   NAS identification attributes (NAS-IP-Address, NAS-IPv6-Address,
   NAS-Identifier) match the source address of packets originating from
   the NAS.  Where a match is not found, an Access-Reject SHOULD be
   sent, and an error SHOULD be logged.

   However, such a check may not always be possible.  Since the
   NAS-Identifier attribute need not correspond to an FQDN, it may not
   be resolvable to an IP address to be matched against the source
   address.  Also, where a NAT exists between the RADIUS client and
   proxy, checking the NAS-IP-Address or NAS-IPv6-Address attributes may
   not be feasible.

   To allow verification of NAS and session identification parameters,
   EAP methods can support the secure exchange of these parameters
   between the EAP peer and EAP server.  NAS identification attributes
   include NAS-IP-Address, NAS-IPv6-Address and Called-Station-Id;
   session identification attributes include User-Name and
   Calling-Station-Id.  The secure exchange of these parameters between
   the EAP peer and server enables the RADIUS server to check whether
   the attributes provided by the NAS match those provided by the peer;
   similarly, the peer can check the parameters provided by the NAS
   against those provided by the EAP server.  This enables detection of
   a rogue NAS.








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<span class="h4"><h4><a class="selflink" name="section-4.3.8" href="#section-4.3.8">4.3.8</a>.  Man in the Middle Attacks</h4></span>

   RADIUS only provides security on a hop-by-hop basis, even where IPsec
   is used.  As a result, an attacker gaining control of a RADIUS proxy
   could attempt to modify EAP packets in transit.  To protect against
   this, EAP methods SHOULD incorporate their own per-packet integrity
   protection and authentication mechanisms.

<span class="h4"><h4><a class="selflink" name="section-4.3.9" href="#section-4.3.9">4.3.9</a>.  Separation of Authenticator and Authentication Server</h4></span>

   As noted in [<a href="http://tools.ietf.org/html/rfc2716" title="&quot;PPP EAP TLS Authentication Protocol&quot;">RFC2716</a>], it is possible for the EAP peer and
   authenticator to mutually authenticate, and derive a Master Session
   Key (MSK) for a ciphersuite used to protect subsequent data traffic.
   This does not present an issue on the peer, since the peer and EAP
   client reside on the same machine; all that is required is for the
   EAP client module to derive and pass a Transient Session Key (TSK) to
   the ciphersuite module.

   The situation is more complex when EAP is used with RADIUS, since the
   authenticator and authentication server may not reside on the same
   host.

   In the case where the authenticator and authentication server reside
   on different machines, there are several implications for security.
   First, mutual authentication will occur between the peer and the
   authentication server, not between the peer and the authenticator.
   This means that it is not possible for the peer to validate the
   identity of the NAS or tunnel server that it is speaking to, using
   EAP alone.

   As described in <a href="#section-4.2">Section 4.2</a>, when RADIUS/EAP is used to encapsulate
   EAP packets, IPsec SHOULD be used to provide per-packet
   authentication, integrity, replay protection and confidentiality.
   The Message-Authenticator attribute is also required in RADIUS
   Access-Requests containing an EAP-Message attribute sent from the NAS
   or tunnel server to the RADIUS server.  Since the
   Message-Authenticator attribute involves an HMAC-MD5 message
   integrity check, it is possible for the RADIUS server to verify the
   integrity of the Access-Request as well as the NAS or tunnel server's
   identity, even where IPsec is not used.  Similarly, Access-Challenge
   packets containing an EAP-Message attribute sent from the RADIUS
   server to the NAS are also authenticated and integrity protected
   using an HMAC-MD5 message integrity check, enabling the NAS or tunnel
   server to determine the integrity of the packet and verify the
   identity of the RADIUS server, even where IPsec is not used.
   Moreover, EAP packets sent using methods that contain their own
   integrity protection cannot be successfully modified by a rogue NAS
   or tunnel server.



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   The second issue that arises where the authenticator and
   authentication server reside on separate hosts is that the EAP Master
   Session Key (MSK) negotiated between the peer and authentication
   server will need to be transmitted to the authenticator.  Therefore a
   mechanism needs to be provided to transmit the MSK from the
   authentication server to the NAS or tunnel server that needs it.  The
   specification of the key transport and wrapping mechanism is outside
   the scope of this document.  However, it is expected that the
   wrapping mechanism will provide confidentiality, integrity and replay
   protection, and data origin authentication.

<span class="h4"><h4><a class="selflink" name="section-4.3.10" href="#section-4.3.10">4.3.10</a>.  Multiple Databases</h4></span>

   In many cases a security server will be deployed along with a RADIUS
   server in order to provide EAP services.  Unless the security server
   also functions as a RADIUS server, two separate user databases will
   exist, each containing information about the security requirements
   for the user.  This represents a weakness, since security may be
   compromised by a successful attack on either of the servers, or their
   databases.  With multiple user databases, adding a new user may
   require multiple operations, increasing the chances for error.  The
   problems are further magnified in the case where user information is
   also being kept in an LDAP server.  In this case, three stores of
   user information may exist.

   In order to address these threats, consolidation of databases is
   recommended.  This can be achieved by having both the RADIUS server
   and security server store information in the same database; by having
   the security server provide a full RADIUS implementation; or by
   consolidating both the  security server and the RADIUS server onto
   the same machine.

<span class="h2"><h2><a class="selflink" name="section-5" href="#section-5">5</a>.  IANA Considerations</h2></span>

   This specification does not create any new registries, or define any
   new RADIUS attributes or values.

<span class="h2"><h2><a class="selflink" name="section-6" href="#section-6">6</a>.  References</h2></span>

<span class="h3"><h3><a class="selflink" name="section-6.1" href="#section-6.1">6.1</a>.  Normative References</h3></span>

   [<a name="ref-RFC1321" id="ref-RFC1321">RFC1321</a>]      Rivest, R., "The MD5 Message-Digest Algorithm", <a href="http://tools.ietf.org/html/rfc1321">RFC</a>
                  <a href="http://tools.ietf.org/html/rfc1321">1321</a>, April 1992.

   [<a name="ref-RFC2104" id="ref-RFC2104">RFC2104</a>]      Krawczyk, H., Bellare, M. and R. Canetti, "HMAC:
                  Keyed-Hashing for Message Authentication", <a href="http://tools.ietf.org/html/rfc2104">RFC 2104</a>,
                  February 1997.




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   [<a name="ref-RFC2119" id="ref-RFC2119">RFC2119</a>]      Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", <a href="http://tools.ietf.org/html/bcp14">BCP 14</a>, <a href="http://tools.ietf.org/html/rfc2119">RFC 2119</a>, March 1997.

   [<a name="ref-RFC2279" id="ref-RFC2279">RFC2279</a>]      Yergeau, F., "UTF-8, a transformation format of ISO
                  10646", <a href="http://tools.ietf.org/html/rfc2279">RFC 2279</a>, January 1998.

   [<a name="ref-RFC2284" id="ref-RFC2284">RFC2284</a>]      Blunk, L. and J. Vollbrecht, "PPP Extensible
                  Authentication Protocol (EAP)", <a href="http://tools.ietf.org/html/rfc2284">RFC 2284</a>, March 1998.

   [<a name="ref-RFC2401" id="ref-RFC2401">RFC2401</a>]      Atkinson, R. and S. Kent, "Security Architecture for
                  the Internet Protocol", <a href="http://tools.ietf.org/html/rfc2401">RFC 2401</a>, November 1998.

   [<a name="ref-RFC2406" id="ref-RFC2406">RFC2406</a>]      Kent, S. and R. Atkinson, "IP Encapsulating Security
                  Payload (ESP)", <a href="http://tools.ietf.org/html/rfc2406">RFC 2406</a>, November 1998.

   [<a name="ref-RFC2409" id="ref-RFC2409">RFC2409</a>]      Harkins, D. and D. Carrel, "The Internet Key Exchange
                  (IKE)", <a href="http://tools.ietf.org/html/rfc2409">RFC 2409</a>, November 1998.

   [<a name="ref-RFC2486" id="ref-RFC2486">RFC2486</a>]      Aboba, B. and M. Beadles, "The Network Access
                  Identifier", <a href="http://tools.ietf.org/html/rfc2486">RFC 2486</a>, January 1999.

   [<a name="ref-RFC2865" id="ref-RFC2865">RFC2865</a>]      Rigney, C., Willens, S., Rubens, A. and W. Simpson,
                  "Remote Authentication Dial In User Service (RADIUS)",
                  <a href="http://tools.ietf.org/html/rfc2865">RFC 2865</a>, June 2000.

   [<a name="ref-RFC2988" id="ref-RFC2988">RFC2988</a>]      Paxson, V. and M. Allman, "Computing TCP's
                  Retransmission Timer", <a href="http://tools.ietf.org/html/rfc2988">RFC 2988</a>, November 2000.

   [<a name="ref-RFC3162" id="ref-RFC3162">RFC3162</a>]      Aboba, B., Zorn, G. and D. Mitton, "RADIUS and IP6",
                  <a href="http://tools.ietf.org/html/rfc3162">RFC 3162</a>, August 2001.

   [<a name="ref-RFC3280" id="ref-RFC3280">RFC3280</a>]      Housley, R., Polk, W., Ford, W. and D. Solo, "Internet
                  X.509 Public Key Infrastructure Certificate and
                  Certificate Revocation List (CRL) Profile", <a href="http://tools.ietf.org/html/rfc3280">RFC 3280</a>,
                  April 2002.

   [<a name="ref-RFC3576" id="ref-RFC3576">RFC3576</a>]      Chiba, M., Dommety, G., Eklund, M., Mitton, D. and B.
                  Aboba, "Dynamic Authorization Extensions to Remote
                  Authentication Dial In User Service (RADIUS)", <a href="http://tools.ietf.org/html/rfc3576">RFC</a>
                  <a href="http://tools.ietf.org/html/rfc3576">3576</a>, July 2003.











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<span class="h3"><h3><a class="selflink" name="section-6.2" href="#section-6.2">6.2</a>.  Informative References</h3></span>

   [<a name="ref-RFC826" id="ref-RFC826">RFC826</a>]       Plummer, D., "An Ethernet Address Resolution
                  Protocol", STD 37, <a href="http://tools.ietf.org/html/rfc826">RFC 826</a>, November 1982.

   [<a name="ref-RFC1510" id="ref-RFC1510">RFC1510</a>]      Kohl, J. and C. Neuman, "The Kerberos Network
                  Authentication Service (V5)", <a href="http://tools.ietf.org/html/rfc1510">RFC 1510</a>, September
                  1993.

   [<a name="ref-RFC1661" id="ref-RFC1661">RFC1661</a>]      Simpson, W., "The Point-to-Point Protocol (PPP)", STD
                  51, <a href="http://tools.ietf.org/html/rfc1661">RFC 1661</a>, July 1994.

   [<a name="ref-RFC2548" id="ref-RFC2548">RFC2548</a>]      Zorn, G., "Microsoft Vendor-specific RADIUS
                  Attributes", <a href="http://tools.ietf.org/html/rfc2548">RFC 2548</a>, March 1999.

   [<a name="ref-RFC2607" id="ref-RFC2607">RFC2607</a>]      Aboba, B. and J. Vollbrecht, "Proxy Chaining and
                  Policy Implementation in Roaming", <a href="http://tools.ietf.org/html/rfc2607">RFC 2607</a>, June
                  1999.

   [<a name="ref-RFC2716" id="ref-RFC2716">RFC2716</a>]      Aboba, B. and D. Simon,"PPP EAP TLS Authentication
                  Protocol", <a href="http://tools.ietf.org/html/rfc2716">RFC 2716</a>, October 1999.

   [<a name="ref-RFC2866" id="ref-RFC2866">RFC2866</a>]      Rigney, C., "RADIUS Accounting", <a href="http://tools.ietf.org/html/rfc2866">RFC 2866</a>, June 2000.

   [<a name="ref-RFC2867" id="ref-RFC2867">RFC2867</a>]      Zorn, G., Aboba, B. and D. Mitton, "RADIUS Accounting
                  Modifications for Tunnel Protocol Support", <a href="http://tools.ietf.org/html/rfc2867">RFC 2867</a>,
                  June 2000.

   [<a name="ref-RFC2868" id="ref-RFC2868">RFC2868</a>]      Zorn, G., Leifer, D., Rubens, A., Shriver, J.,
                  Holdrege, M. and I. Goyret, "RADIUS Attributes for
                  Tunnel Protocol Support", <a href="http://tools.ietf.org/html/rfc2868">RFC 2868</a>, June 2000.

   [<a name="ref-RFC2869" id="ref-RFC2869">RFC2869</a>]      Rigney, C., Willats, W. and P. Calhoun, "RADIUS
                  Extensions", <a href="http://tools.ietf.org/html/rfc2869">RFC 2869</a>, June 2000.

   [<a name="ref-RFC2983" id="ref-RFC2983">RFC2983</a>]      Black, D. "Differentiated Services and Tunnels", <a href="http://tools.ietf.org/html/rfc2983">RFC</a>
                  <a href="http://tools.ietf.org/html/rfc2983">2983</a>, October 2000.

   [<a name="ref-RFC3580" id="ref-RFC3580">RFC3580</a>]      Congdon, P., Aboba, B., Smith, A., Zorn, G. and J.
                  Roese, "IEEE 802.1X Remote Authentication Dial In User
                  Service (RADIUS) Usage Guidelines", <a href="http://tools.ietf.org/html/rfc3580">RFC 3580</a>,
                  September 2003.

   [<a name="ref-IEEE802" id="ref-IEEE802">IEEE802</a>]      IEEE Standards for Local and Metropolitan Area
                  Networks:  Overview and Architecture, ANSI/IEEE Std
                  802, 1990.





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   [<a name="ref-IEEE8021X" id="ref-IEEE8021X">IEEE8021X</a>]    IEEE Standards for Local and Metropolitan Area
                  Networks:  Port based Network Access Control, IEEE Std
                  802.1X-2001, June 2001.

   [<a name="ref-MD5Attack" id="ref-MD5Attack">MD5Attack</a>]    Dobbertin, H., "The Status of MD5 After a Recent
                  Attack", CryptoBytes Vol.2 No.2, Summer 1996.

   [<a name="ref-Masters" id="ref-Masters">Masters</a>]      Slatalla, M. and  J. Quittner, "Masters of Deception."
                  HarperCollins, New York, 1995.

   [<a name="ref-NASREQ" id="ref-NASREQ">NASREQ</a>]       Calhoun, P., et al., "Diameter Network Access Server
                  Application", Work in Progress.







































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<span class="h2"><h2><a class="selflink" name="appendix-A" href="#appendix-A">Appendix A</a> - Examples</h2></span>

   The examples below illustrate conversations between an authenticating
   peer, NAS, and RADIUS server.  The OTP and EAP-TLS protocols are used
   only for illustrative purposes; other authentication protocols could
   also have been used, although they might show somewhat different
   behavior.

   Where the NAS sends an EAP-Request/Identity as the initial packet,
   the exchange appears as follows:

Authenticating peer     NAS                    RADIUS server
-------------------     ---                    -------------
                        &lt;- EAP-Request/
                        Identity
EAP-Response/
Identity (MyID) -&gt;
                        RADIUS Access-Request/
                        EAP-Message/EAP-Response/
                        (MyID) -&gt;
                                               &lt;- RADIUS
                                               Access-Challenge/
                                               EAP-Message/EAP-Request
                                               OTP/OTP Challenge
                        &lt;- EAP-Request/
                        OTP/OTP Challenge
EAP-Response/
OTP, OTPpw -&gt;
                        RADIUS Access-Request/
                        EAP-Message/EAP-Response/
                        OTP, OTPpw -&gt;
                                                &lt;- RADIUS
                                                Access-Accept/
                                                EAP-Message/EAP-Success
                                                (other attributes)
                        &lt;- EAP-Success















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   In the case where the NAS initiates with an EAP-Request for EAP TLS
   [<a href="http://tools.ietf.org/html/rfc2716" title="&quot;PPP EAP TLS Authentication Protocol&quot;">RFC2716</a>], and the identity is determined based on the contents of
   the client certificate, the exchange will appear as follows:

Authenticating peer     NAS                    RADIUS server
-------------------     ---                    -------------
                        &lt;- EAP-Request/
                        EAP-Type=EAP-TLS
                        (TLS Start, S bit set)
EAP-Response/
EAP-Type=EAP-TLS
(TLS client_hello)-&gt;
                        RADIUS Access-Request/
                        EAP-Message/EAP-Response/
                        EAP-Type=EAP-TLS-&gt;
                                              &lt;-RADIUS Access-Challenge/
                                              EAP-Message/
                                              EAP-Request/
                                              EAP-Type=EAP-TLS
                         &lt;- EAP-Request/
                         EAP-Type=EAP-TLS
                         (TLS server_hello,
                         TLS certificate,
                   [TLS server_key_exchange,]
                   [TLS certificate_request,]
                       TLS server_hello_done)
EAP-Response/
EAP-Type=EAP-TLS
(TLS certificate,
TLS client_key_exchange,
[TLS certificate_verify,]
TLS change_cipher_spec,
TLS finished)-&gt;
                        RADIUS Access-Request/
                        EAP-Message/EAP-Response/
                        EAP-Type=EAP-TLS-&gt;
                                              &lt;-RADIUS Access-Challenge/
                                              EAP-Message/
                                              EAP-Request/
                                              EAP-Type=EAP-TLS
                        &lt;- EAP-Request/
                        EAP-Type=EAP-TLS
                        (TLS change_cipher_spec,
                        TLS finished)







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EAP-Response/
EAP-Type=EAP-TLS -&gt;
                        RADIUS Access-Request/
                        EAP-Message/EAP-Response/
                        EAP-Type=EAP-TLS-&gt;
                                              &lt;-RADIUS Access-Accept/
                                              EAP-Message/EAP-Success
                                              (other attributes)
                        &lt;- EAP-Success

   In the case where the NAS first sends an EAP-Start packet to the
   RADIUS server,  the conversation would appear as follows:

Authenticating peer     NAS                    RADIUS server
-------------------     ---                    -------------
                        RADIUS Access-Request/
                        EAP-Message/Start -&gt;
                                               &lt;- RADIUS
                                               Access-Challenge/
                                               EAP-Message/EAP-Request/
                                               Identity
                        &lt;- EAP-Request/
                        Identity
EAP-Response/
Identity (MyID) -&gt;
                        RADIUS Access-Request/
                        EAP-Message/EAP-Response/
                        Identity (MyID) -&gt;
                                                &lt;- RADIUS
                                                Access-Challenge/
                                                EAP-Message/EAP-Request/
                                                OTP/OTP Challenge
                        &lt;- EAP-Request/
                        OTP/OTP Challenge
EAP-Response/
OTP, OTPpw -&gt;
                        RADIUS Access-Request/
                        EAP-Message/EAP-Response/
                        OTP, OTPpw -&gt;
                                                &lt;- RADIUS
                                                Access-Accept/
                                                EAP-Message/EAP-Success
                                                (other attributes)
                        &lt;- EAP-Success







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   In the case where the NAS initiates with an EAP-Request for EAP TLS
   [<a href="http://tools.ietf.org/html/rfc2716" title="&quot;PPP EAP TLS Authentication Protocol&quot;">RFC2716</a>], but the peer responds with a Nak, indicating that it would
   prefer another method not implemented locally on the NAS, the
   exchange will appear as follows:

Authenticating peer     NAS                    RADIUS server
-------------------     ---                    -------------
                        &lt;- EAP-Request/
                        EAP-Type=EAP-TLS
                        (TLS Start, S bit set)
EAP-Response/
EAP-Type=Nak
(Alternative(s))-&gt;
                        RADIUS Access-Request/
                        EAP-Message/EAP-Response/
                        Nak -&gt;
                                               &lt;- RADIUS
                                               Access-Challenge/
                                               EAP-Message/EAP-Request/
                                               Identity
                        &lt;- EAP-Request/
                        Identity
EAP-Response/
Identity (MyID) -&gt;
                        RADIUS Access-Request/
                        EAP-Message/EAP-Response/
                        (MyID) -&gt;
                                               &lt;- RADIUS
                                               Access-Challenge/
                                               EAP-Message/EAP-Request
                                               OTP/OTP Challenge
                        &lt;- EAP-Request/
                        OTP/OTP Challenge
EAP-Response/
OTP, OTPpw -&gt;
                        RADIUS Access-Request/
                        EAP-Message/EAP-Response/
                        OTP, OTPpw -&gt;
                                                &lt;- RADIUS
                                                Access-Accept/
                                                EAP-Message/EAP-Success
                                                (other attributes)
                        &lt;- EAP-Success








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   In the case where the authenticating peer attempts to authenticate
   the NAS, the conversation would appear as follows:

Authenticating peer     NAS                    RADIUS Server
-------------------     ---                    -------------
EAP-Request/
Challenge, MD5 -&gt;
                        RADIUS Access-Request/
                        EAP-Message/EAP-Request/
                        Challenge, MD5 -&gt;
                                                &lt;- RADIUS
                                                Access-Reject/
                                                EAP-Message/
                                                EAP-Response/
                                                Nak (no alternative)

                        &lt;- EAP-Response/Nak
                         (no alternative)
EAP-Failure -&gt;
































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   In the case where an invalid EAP Response is inserted by an attacker,
   the conversation would appear as follows:

Authenticating peer     NAS                    RADIUS server
-------------------     ---                    -------------
                        &lt;- EAP-Request/
                        EAP-Type=Foo
EAP-Response/
EAP-Type=Foo -&gt;
                        RADIUS Access-Request/
                        EAP-Message/EAP-Response/
                        EAP-Type=Foo -&gt;
                                               &lt;- RADIUS
                                               Access-Challenge/
                                               EAP-Message/EAP-Request/
                                               EAP-Type=Foo
                        &lt;- EAP-Request/
                        EAP-Type=Foo
Attacker spoof:
EAP-Response/
EAP-Type=Bar -&gt;

Good guy:
EAP-Response/
EAP-Type=Foo -&gt;
                        RADIUS Access-Request/
                        EAP-Message/EAP-Response/
                        EAP-Type=Bar -&gt;

                                               &lt;- RADIUS
                                               Access-Challenge/
                                               EAP-Message/EAP-Request/
                                               EAP-Type=Foo,
                                               Error-Cause="Invalid EAP
                                                Packet (Ignored)"
                        RADIUS Access-Request/
                        EAP-Message/EAP-Response/
                        EAP-Type=Foo -&gt;
                                               &lt;- Access-Accept/
                                               EAP-Message/Success
                        &lt;- EAP Success










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   In the case where the client fails EAP authentication, and an error
   message is sent prior to disconnection, the conversation would appear
   as follows:

Authenticating peer     NAS                    RADIUS server
-------------------     ---                    -------------
                        RADIUS Access-Request/
                        EAP-Message/Start -&gt;
                                               &lt;- RADIUS
                                               Access-Challenge/
                                               EAP-Message/EAP-Response/
                                               Identity
                        &lt;- EAP-Request/
                        Identity
EAP-Response/
Identity (MyID) -&gt;
                        RADIUS Access-Request/
                        EAP-Message/EAP-Response/
                        (MyID) -&gt;
                                                &lt;- RADIUS
                                                Access-Challenge/
                                                EAP-Message/EAP-Request
                                                OTP/OTP Challenge
                        &lt;- EAP-Request/
                        OTP/OTP Challenge
EAP-Response/
OTP, OTPpw -&gt;
                        RADIUS Access-Request/
                        EAP-Message/EAP-Response/
                        OTP, OTPpw -&gt;
                                                &lt;- RADIUS
                                                Access-Challenge/
                                                EAP-Message/EAP-Request/
                                                Notification
                        &lt;- EAP-Request/
                           Notification

EAP-Response/
Notification -&gt;
                        RADIUS Access-Request/
                        EAP-Message/EAP-Response/
                        Notification -&gt;
                                                 &lt;- RADIUS
                                                 Access-Reject/
                                                 EAP-Message/EAP-Failure
                        &lt;- EAP-Failure
                        (client disconnected)




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   In the case that the RADIUS server or proxy does not support EAP-
   Message, but no error message is sent, the conversation would appear
   as follows:

Authenticating peer     NAS                       RADIUS server
-------------------     ---                       -------------
                        RADIUS Access-Request/
                        EAP-Message/Start -&gt;
                                                  &lt;- RADIUS
                                                  Access-Reject
                        (User Disconnected)

In the case where the local RADIUS server does support EAP-Message, but
the remote RADIUS server does not, the conversation would appear as
follows:

Authenticating peer     NAS                       RADIUS server
-------------------     ---                       -------------
                        RADIUS Access-Request/
                        EAP-Message/Start -&gt;
                                                  &lt;- RADIUS
                                                  Access-Challenge/
                                                  EAP-Message/
                                                  EAP-Response/
                                                  Identity
                        &lt;- EAP-Request/
                        Identity

EAP-Response/
Identity
(MyID) -&gt;
                        RADIUS Access-Request/
                        EAP-Message/EAP-Response/
                        (MyID) -&gt;
                                                  &lt;- RADIUS
                                                  Access-Reject
                                                  (proxied from remote
                                                   RADIUS server)
                        (User Disconnected)












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   In the case where PPP is the link and the authenticating peer does
   not support EAP, but where EAP is required for that user, the
   conversation would appear as follows:

Authenticating peer     NAS                       RADIUS server
-------------------     ---                       -------------
                        &lt;- PPP LCP Request-EAP
                        auth
PPP LCP NAK-EAP
auth -&gt;
                        &lt;- PPP LCP Request-CHAP
                        auth
PPP LCP ACK-CHAP
auth -&gt;
                        &lt;- PPP CHAP Challenge
PPP CHAP Response -&gt;
                        RADIUS Access-Request/
                        User-Name,
                        CHAP-Password -&gt;
                                                  &lt;- RADIUS
                                                  Access-Reject
                        &lt;-  PPP LCP Terminate
                        (User Disconnected)

In the case where PPP is the link, the NAS does not support EAP, but
where EAP is required for that user, the conversation would appear as
follows:

Authenticating peer     NAS                       RADIUS server
-------------------     ---                       -------------
                        &lt;- PPP LCP Request-CHAP
                        auth

PP LCP ACK-CHAP
auth -&gt;
                        &lt;- PPP CHAP Challenge
PPP CHAP Response -&gt;
                        RADIUS Access-Request/
                        User-Name,
                        CHAP-Password -&gt;

                                                 &lt;- RADIUS
                                                 Access-Reject
                        &lt;-  PPP LCP Terminate
                        (User Disconnected)






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<span class="h2"><h2><a class="selflink" name="appendix-B" href="#appendix-B">Appendix B</a> - Change Log</h2></span>

   The following changes have been made from <a href="http://tools.ietf.org/html/rfc2869">RFC 2869</a>:

   A NAS may simultaneously support both local authentication and
   pass-through; once the NAS enters pass-through mode within a session,
   it cannot revert back to local authentication.  Also EAP is
   explicitly described as a 'lock step' protocol. (<a href="#section-2">Section 2</a>).

   The NAS may initiate with an EAP-Request for an authentication Type.
   If the Request is NAK'd, the NAS should send an initial
   Access-Request with an EAP-Message attribute containing an
   EAP-Response/Nak.

   The RADIUS server may treat an invalid EAP Response as a non-fatal
   error (<a href="#section-2.2">Section 2.2</a>)

   For use with RADIUS/EAP, the Password-Retry (<a href="#section-2.3">Section 2.3</a>) and
   Reply-Message (2.6.5) attributes are deprecated.

   Each EAP session has a unique Identifier space (<a href="#section-2.6.1">Section 2.6.1</a>).

   Role reversal is not supported (<a href="#section-2.6.2">Section 2.6.2</a>).

   Message combinations (e.g. Access-Accept/EAP-Failure) that conflict
   are discouraged (<a href="#section-2.6.3">Section 2.6.3</a>).

   Only a single EAP packet may be encapsulated within a RADIUS message
   (<a href="#section-3.1">Section 3.1</a>).

   An Access-Request lacking explicit authentication as well as a
   Message- Authenticator attribute SHOULD be silently discarded
   (<a href="#section-3.3">Section 3.3</a>).

   The Originating-Line-Info attribute is supported (<a href="#section-3.3">Section 3.3</a>).

   IPsec ESP with non-null transform SHOULD be used and the usage model
   is described in detail (<a href="#section-4.2">Section 4.2</a>).

   Additional discussion of security vulnerabilities (<a href="#section-4.1">Section 4.1</a>) and
   potential fixes (<a href="#section-4.3">Section 4.3</a>).

   Separated normative (<a href="#section-6.1">Section 6.1</a>) and informative (<a href="#section-6.2">Section 6.2</a>)
   references.







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   Added additional examples (Appendix A): a NAS initiating with an
   EAP-Request for an authentication Type; attempted role reversal.

Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; neither does it represent that it
   has made any effort to identify any such rights.  Information on the
   IETF's procedures with respect to rights in standards-track and
   standards-related documentation can be found in <a href="http://tools.ietf.org/html/bcp11">BCP-11</a>.  Copies of
   claims of rights made available for publication and any assurances of
   licenses to be made available, or the result of an attempt made to
   obtain a general license or permission for the use of such
   proprietary rights by implementors or users of this specification can
   be obtained from the IETF Secretariat.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive
   Director.

Acknowledgments

   Thanks to Dave Dawson and Karl Fox of Ascend, Glen Zorn of Cisco
   Systems, Jari Arkko of Ericsson and Ashwin Palekar, Tim Moore and
   Narendra Gidwani of Microsoft for useful discussions of this problem
   space.  The authors would also like to acknowledge Tony Jeffree,
   Chair of IEEE 802.1 for his assistance in resolving RADIUS/EAP issues
   in IEEE 802.1X-2001.


















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Authors' Addresses

   Bernard Aboba
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052

   Phone:  +1 425 706 6605
   Fax:    +1 425 936 7329
   EMail:   bernarda@microsoft.com


   Pat R. Calhoun
   Airespace
   110 Nortech Parkway
   San Jose, California, 95134
   USA

   Phone:  +1 408 635 2023
   Fax:    +1 408 635 2020
   EMail:  pcalhoun@airespace.com






























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Full Copyright Statement

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assignees.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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